Eweci? EUROPEAN WIND ENERGY CONFERENCE

FFATN 1997-48 SE0007090

Reserapport samt VKK föredrag

EWEC'97, i Dublin, oktober 1997

sammanställt av Anders Östman och Sven-Erik Thor

eweci? EUROPEAN WIND ENERGY CONFERENCE

FLYGTEKNISKA FÖRSÖKSANSTALTEN THE AERONAUTICAL RESEARCH INSTITUTE OF SWEDEN FFA was founded in 1940. It is the central government agency for aeronautical research in Sweden. Its principal role is to provide scientific support and technical assistance to Swedish Government authorities, to domestic and foreign industries and to other organizations.

Aeronautical research and testing work, as a part of the development of Swedish military and civil aircraft, is the main part of FFA's business. The number of international aerospace customers is, however, now on the increase and the newly installed modern test facilities will enhance this trend. The main areas of research are aerodynamics, structures, acoustics and flight systems. Since the late seventies this experience has also been applied in a wind energy research programme. FFA is also the National Standards Laboratory for Pressure.

FFA has a full range of wind tunnel test facilities from low speed to hyper- sonic speed. The newest wind tunnel - T1500 - is a high performance tran- sonic facility. In addition, FFA has an extensive structures laboratory, a flight research simulator and a new vibro-acoustics laboratory with an anechoic chamber measuring 9 x 10 x 9 (in metres).

FFA is situated close to Bromma airport, seven kilometres north-west of central Stockholm. PLEASE BE AWARE THAT ALL OF THE MISSING PAGES IN THIS DOCUMENT WERE ORIGINALLY BLANK DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. St 0007090— /ÖV

VIND-99/1 Projektrapporter

Reserapport samt VKK föredrag, EWEC-97 i Dublin, oktober 1997 A Östman S-E Thor (eds)

Rapporterna kan beställas från Studsvikbiblioteket, 611 82 Nyköping. Tel 0155-22 10 84. Fax 0155-26 30 44 e-post [email protected] Statens energimyndighet Box 310, 631 04 Eskilstuna Energimyndigheten 1999-09-28

Energimyndigheten

Titel: Reserapport samt VKK föredrag, EWEC-97, i Dublin, oktober 1997

Författare: Anders Östman, ed, Sven-Erik Thor, ed, Flygtekniska försöksanstalten

RAPPORT INOM OMRÅDET VINDKRAFT

Rapportnummer: VIND-99/1

Projektledare: Sven-Erik Thor

Projektnummer: P2939-2

Projekthandläggare på Statens Energimyndighet: Susann Persson

Box 310 • 631 04 Eskilstuna • Besöksadress Kungsgatan 43 Telefon 016-544 20 00 • Telefax 016-544 20 99 [email protected] • www.stem.se FFATN 1997-48

Reserapport samt VKK föredrag

EWEC'97, i Dublin, oktober 1997

sammanställt av Anders Östman och Sven-Erik Thor

ewec?? EUROPEAN WIND ENERGY CONFERENCE

FLYGTEKNISKA FÖRSÖKSANSTAL TEN THE AERONAUTICAL RESEARCH INSTITUTE OF SWEDEN FFATN 1997-48

FLYGTEKNISKA • FÖRSÖKSANSTALTEN THE AERONAUTICAL RESEARCH INSTITUTE OF SWEDEN B.O.Box 11021, S-161 11 Bromma, Sweden Phone +46 8 634 1000, Fax +46 825 34 81 FFATN 1997-48

Sammanfattning

Den Europeiska vindenergiföreningen, EWEA, arrangerar vart tredje år en konferens och utställning, European Wind Energy Conference EWEC. I år, 1997, arrangerades EWEC'97 i Dublin. Vart tredje år arrangerar dessutom Europeiska Unionen EUWEC sin vindenergikonferens. Konfe- renserna är förskjutna ett och ett halvt år inbördes. Det innebär att det i Europa arrangeras vindkonferenser var 18:e månad.

I föreliggande rapport redovisas erfarenheter från EWEC'97. Tyngd- punkten har lagts på att referera det som hände inom de olika forsknings- områdena som VKK är verksamma inom. Erfarenheter från övriga områ- den kommer att sammanställas av Vattenfall på uppdrag av Elforsk. För en fullständig information från konferensen rekommenderar vi därför läs- ning av båda rapporterna. Nästa europeiska vindenergikonferens, EUWEC'99, arrangeras i Nice i mars 1999. Konferensens olika avsnitt har bevakats av följande personer som även har författat texten under respektive avsnitt. 1. Vindunderlag och potentialfrågor Hans Bergström 2. Aerodynamik Anders Björck 3. Dynamik Hans Ganander 4. Funktion och prestanda Jan-Åke Dahlberg 5. Elsystem och reglering Ola Carlson 6. Nätintegration Ola Carlson 7. Buller Sten Ljunggren 8. Acceptansfrågor Karin Hammarlund 9. Stora vindturbiner Sven-Erik Thor FFATN 1997-48

Innehållsförteckning

1 INLEDNING 7

2 ALLMÄNT OM KONFERENSEN 7

3 INVIGNINGEN 8

4 REFERAT FRÅN TEKNIKOMRÅDEN 9

4.1 Vindunderlag och potentialfrågor 9

4.2 Aerodynamik 12

4.3 Strukturdynamik 16

4.4 Funktion och prestanda 17

4.5 Elsystem och reglerteknik 18

4.6 Nätintegration 21

4.7 Buller 22

4.8 Acceptansfrågor 24

4.9 Stora maskiner 25

4.10 Övrigt 25

5 SAMMANSTÄLLNING AV VKK PRESENTATIONER 27 FFATN 1997-48

1 Inledning

Den 6 - 9:e oktober 1997 arrangerades EWEC'97 i Dublin. Konferensen samlade cirka 460 deltagare. Som jämförelse kan nämnas att den förra vindkonferensen i Europa, EUWEC'96 i Göteborg, samlade cirka 600 personer. Erfarenheter som presenterades på konferensen visar att vindkraften ökar kraftigt i de olika europeiska länderna. Under det senaste året har utbyggnaden varit mycket kraftig i ett flertal länder, speciellt i Tyskland. Under 1996 ökade den installerade vindkraften med 1300MW och vid slutet av 1996 fanns det totalt i hela världen ungefär 6100 MW. Tyskland och USA är de länder som har mest vindkraft i världen. Dan- mark ligger på tredje plats och Sverige finns på åttonde plats med 103 MW installerad effekt.

2 Allmänt om konferensen

0 Arets konferens hade samlat färre deltagare än EUWEC'96 i Göteborg. Anledningen var enligt uppgift att konferenslokalerna på Dublin Castle inte klarade av mer än cirka 300 deltagare. Arrangören hade därför varit sparsamma med annonsering av arrangemanget. Det var 26 olika länder representerade. Nedanstående tabell visar deltaga- rantal från några olika länder. England 73 Danmark 68 Irland 58 Tyskland 46 Holland 40 Spanien 24 USA 23 Sverige 22 Japan 18 Grekland 17 Finland 16 Frankrike 11 samt ett mindre antal från Belgien, Kanada, Kina, Estland, Italien, Malta, Mexico, Marocko, Norge, Polen, Portugal, Sydafrika, Ryssland och Schweiz. Totalt var 460 personer anmälda till konferensen. FFATN 1997-48

Totalt presenterades 104 muntliga föredrag och 107 "posters". Översikt- ligt program för konferensen framgår av Figur 1. Vindkraftskonsortiet presenterade 13 olika föredrag och "posters". I anslutning till konferenslokalerna fanns en utställning. Utställningen av "posters" fick speciell uppmärksamhet vid konferensen genom det "poster award" som delades ut till bästa "poster".

3 Invigningen

Konferensen invigdes av Irlands Energiminister Joe Jacobs, (det lyck- ades vi inte med i Göteborg). Han redogjorde för det ambitiösa program

som man har i Irland för att introducera CO2-fri elproduktion. Vindkraf- ten kommer att vara en viktig del i en sådan politik. Andelen vindkraft är för närvarande liten, men den ökar snabbt. I juni -97 hade man 24.5 MW installerad effekt och den kommer vid årsskiftet 97/98 att vara 50 MW. Arrangören hade för invigningen engagerat ett flertal "tunga" namn inom energiområdet: Joanna Tachmintzis från EU-kommissionen redogjorde för den strategi som kommissionen har för att introducera förnybar elproduktion i Euro- pa. Målet är att vi skall ha 12% år 2010, i dag är andelen 6%. För att sti- mulera denna introduktion kommer kommissionen inom en snar framtid att presentera ett "white paper" om energi samt en tillhörande "plan for action". I sitt anförande pekade hon på några frågor som är viktiga vid den framtida utvecklingen av vindkraften: • frågor kring tillgången till elnätet måste lösas • rättsliga aspekter vid introduktion av vindkraften i landskapet • viktigt att se samband mellan vindkraftstillverkning och arbetstillfällen

• minskning av CO2-utsläpp med 15% mellan åren 1990 och 2010 Den sista punkten kommer att vara EU:s krav vid FN-konferensen om miljöpåverkan som kommer att arrangeras i december 1997 i Kyoto, Ja- pan. M. Jefferson, Deputy Secretary General, World Energy Council, menade att kemin bakom växthuseffekterna förstås till fullo och att det är av stor vikt att minska CO2-utsläppen i världen. Ett citat - Om kineserna fortsät- ter att öka sin kolanvändning i nuvarande takt kommer man om 15-20 år att få säga "good-bye" till sina risskördar. FFATN 1997-48

Birger T. Madsen gav en sammanställning av vindkraftens status och ut- byggnad just nu. Han visade att EU:s planer 1990 var att det år 2000 skulle finnas 4000 MW installerad vindkraft. Det målet har vi redan i dag uppnått. Utbyggnaden har således gått betydligt snabbare än vad man för- utsåg på EWEC'91 i Amsterdam. Madsen pekade på de trender som han såg för framtidens vindkraftverk: • uppskalning kommer att fortsätta • direktdrivna generatorer kommer att ta marknadsandelar • "offshore" och stora maskiner kommer i framtiden • prognos för 2001 är att det finns 9406 MW • aggregat av storleken 500 - 1000 kW kommer att vara marknadsle- dande under de kommande fem åren Sammanfattningsvis kan vi konstatera att de presentationer som gjordes vid invigningen var mycket optimistiska om vindkraftens fördelar och möjligheter i framtiden. Kuriosa. Ett intressant påpekande gjordes av G. van Kuik avseende de engelska orden "windmill" och "wind power". Observera att det finns ett mellanslag i wind power och att så ej är fallet i windmill. Detta beror enligt lexikonet på att windmill betraktas som något vardagligt och inarbe- tat. G. vanKuik tyckte därför att det nu är dags att skriva ihop wind power till ett ord!

4 Referat från teknikområden

VKK var representerade vid konferensen med 13 föredrag och posters. I denna rapport finns en sammanställning av dessa. Dessutom bifogas någ- ra föredrag med anknytning till VKK:s verksamhet. Samtliga föredrag kommer senare att presenteras i proceedings från konferensen. Nedan görs en sammanställning av olika "teknikområden". Erfarenheter från övriga områden kommer att sammanställas av Vattenfall Utveckling AB på uppdrag av Elforsk. För fullständig information från konferensen rekommenderar vi därför läsning av båda rapporterna.

4.1 Vindunderlag och potentialfrågor Två sessioner med föredrag tillsammans med en postersession behandlade ämnet vindresurser. Presentationerna, sammanlagt 26 st, kan indelas i följande huvudgrupper: • vindklimat, potential och modeller • instrument och kalibrering FFATN 1997-48

Generellt kan sägas att många presentationer behandlade förhållandena i komplex och heterogen terräng, medan off shore endast berördes i ett par fall. Kanske beror detta delvis på den felaktiga föreställningen att vind- förhållandena till havs är 'snälla' och okomplicerade. Förhållandevis mycket arbete verkade också ha lagts ned på s.k. 'site calibration', d.v.s. hur man ska kunna verifiera energiproduktion mot en referens då man har heterogen terräng. Däremot sades tyvärr inget om bättre metoder och modeller för att kunna optimera valet av platser för vindturbiner i en heterogen omgivning med mycket variabla vindförhållanden, syftande till att turbinerna skall kunna placeras så att de producerar så mycket energi som möjligt.

Vindklimat, potential och modeller Resultat från studier av vindpotential rapporterades från tre områden, Östersjön, Irland, samt Kolahalvön. I det senare fallet har enbart den danska WASP modellen använts tillsammans med mer eller mindre brist- fälliga data från 12 meteorologiska stationer, medan i fallet Irland en intressant metod använts där man kombinerar WA P körningar för den småskaliga och detaljerade vindkarteringen, med körningar med en mycket mer fullständig mesoskalemodell för det atmosfäriska gränsskik- tet, den s.k. KAMM-modellen från Karlsruhe i Tyskland. Denna modell kan på ett mera realistiskt sätt än WASP modellera vindfältet med en rum- supplösning av 5 km. Detta mesoskaliga vindfält kan sedan användas som underlag för en mer detaljerad kartläggning med WASP. Metoden att utnyttja en numerisk mesoskalemodell bör ge en betydligt bättre vindpoten- tialkartering än vad som är möjligt om enbart WASP används. Vindkarte- ringen över Östersjön har gjort enbart med hjälp av en mesoskalig gräns- skiktsmodell, MlUU-modellen från Meteorologiska institutionen i Upp- sala, och syftade till att få en första uppfattning om vindpotentialen över öppet hav. MlUU-modellen kan, till skillnad från den betydligt enklare WASP, generera de klimatologiskt betydelsefulla vindmaxima på relativt låg höjd som frekvent observerats i Östersjöområdet. I en poster redogjordes för observationer av dessa vindmaxima (low level jets) samt visades ett par exempel på vindfält med MlUU-modellen från fall med sådana vindmaxima. En presentation redogjorde för en sensitivitetsstudie av hur väl vindklimatet bestämt med WASP stämmer överens med mätningar i mycket komplex terräng i Portugal, samt vad upplösningen i det tillgängliga di- gitala kartmaterialet betyder för resultatet. Det visade sig att om man helt oförbehållslöst använder mätningar från en plats i ett bergsområde för att bestämma vindklimatet på en annan plats kunde felen uppgå till ±40- 50%. Om man däremot valde sin referensstation så att dess komplexitets- grad avseende terrängens utseende stämde överens med vad man hade på den plats man skall uppskatta vindklimatet på, så kunde felen nedbringas

10 FFATN 1997-48

till storleksordningen ±5-10%. Detta är ju dock fortfarande stora fel, och med tanke på att man i de flesta praktiska fall har mycket få mätstationer att tillgå och därför egentligen i praktiken inte alls har några möjligheter att ställa krav på platserna, så är fel av storleksordningen flera tiotals pro- cent nog att befara i många fall. Slutsatsen får nog bli att bättre modeller måste till för att man ska kunna bedöma vindpotentialen i heterogen ter- räng. Att utnyttja modeller som inte är gjorda för att användas i bergsom- råden är inte att rekommendera. Generellt avseende vindar i komplex och heterogen terräng kan nämnas tre studier. Från mätningar på Shetlandsöarna rapporterades om analyser av hur de statistiska egenskaperna hos det turbulenta vindfältet modifieras i komplex terräng jämfört med mer homogena förhållanden, speciellt då i termer av spektra och koherens. I en annan presentation redogjordes för vissa resultat vad avser medelvind och turbulensstatistik i komplex ter- räng på Marmari i Grekland. En analys från en fjälldal vid Suorva i Lappland visade att det inte enbart är på högfjället som en god vindpotential kan förekomma. Flera presentationer handlade om vindförhållandena och vindenergi generellt i arktisk miljö där problem med nedisning kan förekomma. Speciellt har Finland här varit aktivt. I en studie redogjordes för ett första försök att kartera var i Europa nedisning kan befaras.

Instrument och kalibrering Inom detta område gavs 5 presentationer. I två arbeten redovisades studier rörande kalibreringar av skålkorsanemometrar. I det första behandlades erfarenheter av rekommendationer av MEASNET, ett nätverk av mätin- stitut. I det andra arbetet redogjordes för en jämförelse mellan kalibreringar av en anemometer som skickats runt i världen till olika vindtunnlar. Båda studierna visar att en kalibreringsnoggrannhet på bättre än 1% inte alltid är så lätt att få. En presentation berörde också problemställ- ningar vad gäller att jämföra prestanda för olika typer av skålkorsanemo- metrar och föreslog en möjlighet att klassificera anemometrarna. En typ av anemometrar som under senare år blivit allt vanligare är de s.k. ultraljudsanemometrarna. Dessa mäter vindens tre komponenter genom att mäta tiden det tar för en ultraljudspuls att färdas mellan tre par av sän- dare-mottagare. Två presentationer handlade om denna typ av anemometrar. Problemet med dem är att de ger en omfattande aerodynamisk stör- ning av vindfältet runt instrumentet, vilket får till följd att mätningarna måste korrigeras med så mycket som upp till 10% på vindhastigheten i vissa vindriktningssektorer för att bli korrekta. Sådana korrektioner är dock möjliga att göra efter kalibrering i vindtunnel, vilket också redovisas i ett arbete där sådana kalibreringar genomförts. En fördel med ultra- ljudsanemometrar jämfört med skålkorsanemometrar är, frånsett att de

11 FFATN 1997-48

inte har några rörliga delar som slits, att de efter korrektionen inte enbart ger vinden i horisontalplanet utan alla tre komponenterna, dessutom med en frekvensupplösning av hela 20 Hz.

4.2 Aerodynamik Aerodynamik presenterades under två muntliga sessioner samt med 12 posters. Presentationerna kan indelas i följande ämnesområden. • Profildesign • Navier-Stokes metoder och potentialströmningsmodeller. • Buller • "Dubbel-stall" • Dynamisk stall • Is-problematik Nedan följer kommentarer kring några av presentationerna.

Profil och bladdesign Risö visade på en testrigg för profilprov som de använder i den danska VELUX-tunneln samt på exempel på utveckling av profiler. Peter Fugle- sang visade hur han använt en generell numerisk optimeringsmetod i kombination med profilberäkningsprogrammet XFOIL för ta designa profiler. Han visade ett exempel med en 24 % t/c "koncept-profil". Denna hade optimerats för att erhålla maximal strukturell styvhet med bivillkor för stallkarakteristik, design-Cl och glidtal. Optimeringsförfarandet visar på stora möjligheter att ta fram bra profiler skräddarsydda för vindkraftverk. Det är uppenbart att Danmark (Risö) satsar på att ta fram skräddar- sydda profiler för framtida danska bladprojekt. City University i London har sedan flera år ett EU-projekt på gång tillsammans med Ecotecnica i Spanien och numera i en andra fas även med LM Glasfiber. Projektet handlar om "Air-jet vortex generators". Virvel generatorer (Vortex generators, VG) har redan förut använts för att få en profil som utan VG's skulle få avlöst gränsskikt att vidmakthålla anliggande strömning, (avlöst strömning ger högre motstånd och förlust av lyftkraft). Normalt används små deltavingar som ställs upp vinkelrätt mot ytan (höjden på vingarna är av samma storlek som gränsskiktet, typiskt ca 20 mm för en 20-metersvinge). Problemet med dessa VG's är att de orsakar ett "paracitmotstånd" vid låga vindar då de egentligen inte behövs. För att reglera effekten av VG's skulle man vilja kunna justera deras höjd. Projektet "Air-jet vortex generators" går ut på att man blåser ut luft genom små hål i vingen. Luftströmmen som blåses ut orsakar en virvel som blandar ner energirik strömning från utanför gränsskiktet ner till ström-

12 FFATN 1997-48

ningen vid bladytan på samma sätt som mer konventionella VG's. Vid lå- ga vindar blåses ingen luft ut och vingen blir som en vinge utan VG's. När man närmar sig stall kan man börja blåsa ut luft och fördröja stallen. Vid stall är förhoppningen att man skall kunna reglera stallförloppet och på så sätt effekten. Tanken är att man skall få en stall-reglerad turbin med samma gynnsamma effektkurva som en pitch-reglerad. Man har utfört 2- D vindtunnelprov som visar att "Air-jet vortex generators" fungerar. Inle- dande fullskaleprov har gjorts. Man hade hål vid radiella stationer mellan 55-75% r/R. Fullskaleproven visade att man kunde höja max effekt från ca 140-160 kW för den specifika turbinen med hjälp av blåsning. Vid dessa försök hade man en fläkt i navet för att erhålla tillräckligt högt tryck till utblåset. För att driva denna fläkt åtgick emellertid en relativt stor effekt och mycket av effektvinsten försvann. Förhoppningar finns dock att man skall kunna designa tilluftkanlen i vingen bättre och att det fungerar bättre med utblås på större radier. Man kommer dock även att se hur långt man kan nå genom att använda enbart "ramtrycket". Sammanfattningsvis så kan man säga att man ännu inte lyckats uppnå hela målet att ge en stall- reglerad turbin lika stor energifångning som en pitch-reglerad turbin med samma radie. Projektet visar ändå på goda möjligheter till både god reglering och förhöjning av verkningsgraden med hjälp av "Air-jet vortex generators".

Navier-Stokes metoder och potentialströmningsmodeller Nio presentationer handlade om beräkningar med Navier-Stokes tidsme- delvärdesbildade ekvationer och beräkningar med potentialmetoder. Jäm- fört med konferenser tidigare år kan man se att framsteg görs på området för Navier-Stokes metoder och att presentationer med Navier-Stokes lös- ningar är fler än presentationer med potentialströmningsmetoder. Man redovisar nu N-S beräkningar på hela turbinblad. Utifrån jämförelse med 2D beräkningar och 3D beräkningar på blad hoppas men t.ex. att kunna ta fram semi-empiriska korrektionsmetoder att användas i BEM-beräkningar (Hansen, Sörensen, m.fl.). Samtliga deltagare påtalade dock att utveckling av turbulensmodeller fortfarande behövs för att kunna prediktera Cl och Cd i "post-stall"-området" med större säkerhet.

Buller Tre presentationer hade anknytning till buller. Resultat från EU JOULE JU projekten DRAW och STENO presenterades. I projektet DRAW skall man förbättra/utveckla existerande beräkningsmetoder för bullerberäk- ningar. Man utför mätningar av buller från 2D profiler i en ekofri vindtunnel. Man mäter gränsskiktsparametrar (t.ex. instationära trycknivåer på profilytan) samtidigt som man mäter buller med sofistikerade "antenner". Utifrån mätningarna skall man ta fram samband mellan buller och gräns- skiktsparametrar. Man har även studerat buller p.g.a. turbulens i den an-

13 FFATN 1997-48

strömmande luften (inflow turbulens noise). Dessa senare mätningar veri- fierar till stor del de teoretiska modeller som man utvecklar. T.ex. har man visat och verifierat hur man kan konstruera profiler som har en ut- formning (främst runt framkanten) så att "inflow turbulence noise" kan minskas. ISTENO har man gjort fullskaleprov med "serrated trailing edges" Detta innebär att bakkanten är sågtandad. Enligt teorin skall en sådan bakkant kunna ge en reduktion av bullret genererat av det turbulenta gränsskiktet (trailing edge noise) med flera dB. Dessa teorier har också bekräftats vid 2D vindtunnelprov. De fullskaleprov man utfört visar att man, relativt ett blad utan sågtands-bakkant, erhåller en reduktion av bullret för vissa frekvenser men för andra frekvenser så ökar istället bullret. Proven är dock ännu inte slutförda och resultaten än så länge preliminära. Kristian S. Dahl hade en poster som visar att man börjat med "Computational Aero-Acoustics" på Risö. Detta innebär att man beräknar ljudet direkt från lösningen av en instationär CFD beräkning. Man har be- räknat ljudsignaturen från en profil vid hög anfallsvinkel då periodiska avlösningsfenomen uppträder. Man har dock problem med att uppnå sta- bilitet i ljudberäkningarna.

Dubbel-stall Många stall-reglerade turbiner (de med NACA 63-200 profiler) uppvisar multipla nivåer på effekten i stallområdet. Detta är inte önskvärt då man helst vill ha en entydig vind-effekt kurva. Det är t.ex. annars svårt att uppskatta energiproduktionen från vindkraftverken. Man har länge sökt en rimlig förklaring på detta fenomen. Risö har även funnit att flera nivåer på Cl är möjligt vid genomförda vindtunnelprov, såväl 2D prov som prov med ett icke-roterande hela blad. För konstant anfallsvinkel kan ström- ningen ändra sig och resultera i olika Cl-nivåer. (Detta är inte repetitivt på samma sett som statisk stall-hysteres). Genom analys med CFD kod har man funnit att fenomenet troligen har att göra med gränsskiktomslag. NACA 63-215 profilen är mycket känslig för ifall omslag till turbulent gränsskikt sker (eller inte sker) innan laminär-separation och en laminär gränsskikts-bubbla utbildar sig. Ifall laminär-separation sker så är ström- ningen sedan mycket beroende på hur snabbt omslag till turbulent ström- ning sker i bubblan. Det är troligt att små störningar i omslagsprocessen (t.ex. små radiella flöden i gränsskiktet) kan påverka ifall man får en kort bubbla och anliggande strömning efter bubblan eller ifall omslag sker så sent att bubblan inte återanligger vilket orsakar kraftig avlösning och ett lägre CL.

Vad är det då för vits med att man kommit på en trolig anledning till "dubbel-stall"? Jo, man kan i framtiden se till att inte använda denna typ

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av profiler med en funktion av anfallsvinkeln så att lyftkraften blir så ex- tremt känslig för störningar i omslagsprocessen.

Dynamisk stall Herman Snel visade hur man matematiskt kan simulera det fenomen att lyftkraften får en oscillerande del vid kraftigt avlöst strömning. Genom att bestämma konstanter i den (icke-linjära) matematiska modellen fick han god överensstämmelse med resultat från vindtunnelprov med dynamisk stall där "oscillerande överslängar" av lyftkraften uppmätts. Det är ännu osäkert huruvida dessa "oscillerande överslängar" har någon betydelse för lasterna på bladet. Detta skall utredas. En stor finess är att den modell som Snel redovisade kan läggas till existerande semi-empiriska modeller för dynamisk stall av den typ som bl.a. använts i VIDYN.

FFA och Teknikgruppen visade i en presentation (Computations of Aero- dynamic Damping for Blade Vibrations in Stall) på hur den beräknade aerodynamiska dämpningen av bladsvängningar är beroende av hur man modellerar effekten av dynamisk stall. Man visade även hur viktigt bladets utböjningsformer är. Den aerodynamiska dämpningen är beroende av hur anfallsvinkeln för ett bladelement ser ut som funktion av tiden under det att bladet svänger. Relativt små ändringar i orienteringen av bladets huvudtrögehetsaxel har en stor inverkan på dämpningen av svängningar. Detta visar hur viktigt det är att ha turbinens strukturella beteende väl be- skrivet, men visar även på möjligheterna att medelst kopplingar mellan olika typer av rörelser skapa bättre aerodynamisk dämpning.

Is-problematik Henry Seifert visade prov på nedisning av blad. Man har gjutit av ett flertal blad som varit nedisade och gjort modeller som man kört 2D-prov i vindtunnel med. Detta har lett till en omfattande databas med Cl, Cd och Cm för nedisade profiler. Per Völund från Risö redovisade resultaten från ett JOULE projekt med syftet att studera is-inducerade laster. Man visade hur man ofta får en kraftig minskning av den elektriska effekten. Det finns dock fall då den kan öka i stall-området. Detta beror på att bladytan ökar utan att Cl och Cd "förstörs" i samma omfattning. Man har ägnat stor tid att studera de dynamiska lasterna. Man har studerat frekvensspektra av bladrotmoment för fall då blad varit fria från is respektive nedisade. Man hävdade att egenviktsinflytandet av isen var försumbart. Vid frekvensen för den första "edge-moden" kan man se en stor förhöjning av lastnivåerna för nedisade blad. Vid 13 m/s har en turbin visat på kraftiga vibrationsnivåer trots att bladens uppvärmningssystem var igång.

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4.3 Strukturdynamik Vad muntliga presentationer och posters beträffar var mitt intresse särskilt inriktat på att följa upp strukturdynamik och verifieringar av beräknings- program. I det följande ges en personlig bild av vad konferensen gav på dessa områden. Man konstaterade emellertid snabbt att konferenspro- grammet inte innehåller någon explicit rubrik som berör detta. Sett över några år innebär detta en märkbar förändring. Beräkningsprogram och strukturdynamik ingår emellertid som en basresurs för studier på många andra områden inom vindkrafttekniken. Utveckling av aerodynamik som beaktar effekter av strukturdynamik, m.a.o. aeroelasticitet är ett sådant område som i dag är mycket aktuellt. Studier av laster på vindkraftaggre- gat och hur de påverkas av vaksituationer förutsätter också strukturmo- deller. Frågor kring extremlaster och utmattning likaså. VKK är redan in- volverade i de flesta av de projekt med sådan inriktning, som presenterades.

Två presentationer förtjänar ändå att nämnas explicit. Den första är To- mas Kruegers "Reduction of fatigue loads on wind energy converters by advanced control methods". Förutom att han presenterade möjligheterna med reglering för att behärska aggregatdynamiken, är det värt att notera det behov som uppstår i samband med att man i Tyskland i ökande grad lokaliserar aggregat inomlands. Detta kräver högre tom och då blir torn- dynamiken en viktig fråga. Den andra presentationen, som jag vill nämna, gjordes av Spyros Voutsi- nas, i vilken han mycket proffsigt presenterade det nyutvecklade beräk- ningsprogrammet GAST. På sikt är målet att detta skall inkludera både komplett struktur och moderna modeller av aerodynamik. Strukturmo- dellen utgörs av en FE-beskrivning, som normalt leder till några hundra frihetsgrader. Det finns planer på att programmet framöver skall finnas allmänt tillgängligt på nätet ("public domain"). Vid intervju framkom att finansiering av detta återstår, och att planerna är inom storleksordningen ett år. I dag sker uttestning av programmet på kraftfulla arbetsstationer. Utvecklingen av vindkraftteknologin i framtiden togs upp av Peter Hjuler Jensen vid konferensens sista presentation " Wind turbine technology in the 21:st century". Förutom att han blickade bakåt och konstaterade hur svårt det var att för 10-15 år sedan kunna förutsäga situationen i dag och att det sannolikt är lika svårt med förutsägelser i dag, konstaterade han att utvecklingen mot konkurrenskraftigare aggregat leder till lättare och flexiblare aggregat. Detta ställer ökade krav på verifierade och tillförlitli- ga "Design Codes". Andra ingredienser i utvecklingen, som han betona- de, var variabelt varvtal, normer och säkerhetsmarginaler för design.

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4.4 Funktion och prestanda FO.l Windfarms in hostile terrain Ett EU projekt som började 1/1 96 med Garrad & Hassan som koordina- tor. Syftet är att kartlägga belastning i farmerna samt registrera extrem- förhållanden. Problem med väderförhållanden blixtnedslag m.m. har gjort att man ej fått igång mätningar. Man hoppas kunna mäta under vintern 97/98. Projektet avslutas 1988. Under projekteringen konstaterade man att olika standarder stämde dåligt överens samt att lite hjälp erbjöds.

FO.2 How complex can a power curve measurement be, Helmut Klug Extrapolation av vindhastighet Redovisar en metod att extrapolera vindhastigheten till navhöjd som kan användas för stora turbiner för att hålla kostnaden för masten nere. Ge- nom att kombinera information från vindhastighet och temperatur från flera höjder kunde en noggrannare extrapolation göras. Metoden fungerar ej nära kust samt under vissa väderförhållanden. Då anpassningen ej blir så bra förkastas data. Alternativ kalibrering av matplats Metoden går ut på att kalibrera en matplats genom att använda vindhas- tighetsgivaren på maskinhustaket vid stillastående turbin. Platsfelet på maskinhustaket kartläggs m.h.a lasermätning på modell i vindtunnel (utan blad!?) Vissa experimentella jämförelser med fullskala visar att man kan få hygglig överensstämmelse.

FO.3 Comparing the Power Performance Results by Using the Nacelle and the Mast Anemometer, Ionassis Antonio Man har undersökt hur sambandet mellan vindhastighet på maskinhustaket och fri vindhastighet påverkas av ändringar på turbinen, bladvinkel virvelgeneratorer girfel m.m. Grundsambandet bestäms med vindar från en fri sektor. Detta samband används sedan för att bestämma vind-effektkurvan med vindar från en sektor med stora hinder (kolupplag, kolkraftverk). Man konstaterar att sambandet ändras då man gör smärre ingrepp med turbinen samt att metoden med att använda vindhastighet på masinhu staket endast kan använ- das för att verifiera vindeffektkurvan för en turbin då man får ett identiskt samband som för referensturbinen. Om sambandet avviker vet man inget.

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F0.4 Vestas Experience with Offshore Installation Vestas har installerat 10(19?) turbiner V39-500 kW på Tun0 Knob. Man konstaterar att intresset för havslokalisering bland kraftföretag och andra intressenter svängt helt om i Danmark det senaste halvåret och att intresset nu är mycket stort. Modifieringar på själva turbinen, föranlett av havslokalisering, ökade kostnaderna med ca 5%. Man kompletterade turbinen med en extra kran på maskinhustaket. Varvtalet kunde ökas från 30 till 33 rpm eftersom bullerkraven är lägre till havs. Tillgängligheten är hittills något lägre än för motsvarande landbaserade aggregat p.g.a. att man ej kan komma åt turbinerna då våghöjden är >lm samt p.g.a. vissa fågelstudier då turbinerna stoppats. Övriga erfarenheter är att turbulensen är lägre, produktionen ca 25 % högre och kostnaderna högre men ändå 25 % lägre än förväntat. Kostnaderna för nätanslutning och fundament uppvisar de största skillna- derna gentemot landbaserade aggregat. Man konstaterar att kostnaderna för fundamentet är stora, men att de ej är så markant beroende av storleken, vilket gynnar stora aggregat till havs.

FO.5 Condition monitoring of wind farms using 10 minute average Syftet är att förbättra produktionen i vindfarmer genom att använda SCADA (Supervisory Control and Data Acquisition). Förbättringen åstadkoms genom att försöka sortera bort yttre faktorer, smalare sektorer samt använda vindhastigheten som skalfaktor. Metoden används för att detektera olika fel i bladvinkel, växellåda, girfel etc. Verkar fungera bra.

FO.6 Assessment of power performance measurement and evaluation in complex terrain. Undersökning av inverkan på prestanda från olika parametrar som är för- knippade med komplex terräng. Analysen som huvudsakligen utförs genom "mångvariabelanalys" visar hur olika parametrar inverkar. Arbetet ingår som en del i EWTS-II och syftar till att ta fram en teknisk bas för fortsatt standardiseringsarbete.

4.5 Elsystem och reglerteknik Konferensen hade avdelningarna Dynamics & Control, Electrical Com- ponents och Grid Integration som behandlade el- och reglerteknik. Det allmänna intrycket är att variabelt varvtal kommer att dominera på sikt, p.g.a. att nätkvalitén och bullret måste tas i beaktning. Vidare så verkar merparten av deltagarna vara överens om att självkommuterade om-

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riktare, vilka använder sig av IGBT-ventiler, kommer att användas och att de nätkommuterade omriktarna kommer att fasas ut på sikt. Inom området elektriska komponenter presenterades fyra artiklar som behandlade direktdrivna generatorer, två som berörde aktiva filter, en om vindkraftverkets jordning, två som visade på bromsning av en asynkrongenerator vid nätbortfall, en på skydd mot blixtnedslag och en behandlade optimal storlek av transformatorer. T. Hartkopf, M. Hofmann och S. Jöckel med artikeln "Direct-drive generators for megawatt wind turbines" hade undersökt olika typer av magnetisering i kombination med olika typer av likriktare för direktdrivna generatorer. Resultaten visade att permanentmagnetiserade generatorer är lät- tare, effektivare och billigare än elektriskt magnetiserade. Vidare visades att likriktning med IGBT-omriktare har en fördel över likriktning med di- oder. Den senare fördelen är inte lika övertygande som den med PM magnetisering. I artikeln "Direct drive, geared drive, intermediate solution-comparison of design features and operating economics" av G. Boehmeke och R. Bolt visades på klara fördelar för den konventionella växlade drivlinan med asynkrongenerator. Ekonomiska fördelar kan bara visas för den direktdrivna generatorn om mycket höga felfall är antagna för det växlade alternativet. En kommentar till resultaten är att de höga kostnaderna för det direktdrivna alternativet har sin grund i att den direktdrivna generatorn var en traditionellt elektriskt magnetiserad synkrongenerator. Från VKK:s sida presenterades två artiklar, A. Grauers, O. Carlson, E. Högberg, P. Lundmark, M. Johnsson och S. Svenning "Tests and design evaluation of a 20 kW direct-driven permanent magnet generator with a frequency converter" och J. Svensson "The rating of the voltage source inverter in a hybrid wind park with high power quality". Den först nämnda beskrev provmetoder, provresultat och beräkningar av förluster för en 20 kW PM direktdriven generator. Provresultaten visade på goda elektriska prestanda, men då den provade generatorn är en prototyp re- kommenderades några mindre justeringar för att uppnå märkeffekten 20 kW. J. Svensson visade i sin artikel på möjligheterna att med en IGBT- växelriktare aktivt filtrera de övertoner som finns på nätet samtidigt som växelriktaren överför vindkraftverkets effekt ut på nätet. A. Larson visade på sina resultat från arbetet inom Elforsk med artikeln "Optimal size of transformers" att den optimala skenbara effekten på en transformator är detsamma som generatorns märkeffekt eller något lägre. En intressant artikel, J. H. Carstens, "Active filters for power-smoothing and compensation of reactive power and harmonics", behandlades aktiv

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effekt utjämning och aktiv filtrering av ström övertoner från en vindpark. Effektpulsationer uppstår mellan asynkrongeneratorn och nätet för ett vindkraftverk med ett konstantvarvtal. Dessa effektpulsationer kan uppgå till 20% av märkeffekten. Genom att använda sig av ett energilager kan omriktaren leverera eller konsumera aktiv effekt och på så sätt jämna ut effektfluktuationerna på nätet. Den mest intressanta energilagringsprinci- pen som artikeln beskriver är supraledande spolar. Under rubriken Dynamics & Control kunde man finna 5 muntliga presentationer och 9 posters. Ytterligare några reglertekniskt inriktade bidrag återfanns under övriga rubriker. Flera presentationer berörde reglering av aggregat med variabelt varvtal, som ju är den inriktning som studerats inom svensk vindenergiforskning. I ett bidrag från Darmstadt, Tyskland, använde man sig av den gamla väl- kända w2 metoden som för länge sedan prövats på Hönö. I simuleringar har man studerat effekttransienter vid övergången till märkeffekt, utan att egentligen komma med några slutsatser som inte var kända sedan tidigare. Sessionens ordförande Bill Leithead presenterade en artikel, "A robust two-level control design approch to variable speed stall regulated wind turbines" om en designmetod för aktiv stall reglering som ökar stabili- tetsmarginalerna jämfört med en traditionell regulator. Metoden påminner mycket om den kaskadreglering som presenterades i Thommy Ekelunds avhandling. Man har en inre snabb reglering av varvtalet och en lång- sammare yttre för att ta hand om effekten. Priset för ökad robusthet är försämrad prestanda, det vill säga större effektvariation.

Från Risö kom intressanta praktiska resultat där man utrustat ett standar- daggregat ifrån Vestas med variabelt varvtal med hjälp av en Sami Star ifrån ABB i artikel "Experimental investigation of combined variable speed/variable pitch controlled wind turbines" av H. Bindner, A.V. Rebs- dorf och W. Byberg. Tyvärr visade det sig att effekten under märkvind försämrades vid variabelt varvtal. Detta antogs bero på att den verkliga effekt/varvtals karakteristiken avvek ifrån den som antagits vid reglerde- signen. En personlig reflektion är att Samin drar en hel del effekt, vilket konstaterats tidigare för NWP 400. I övrigt så handlade det bland annat om bladvinkelreglering. Vid Aal- borgs universitet hade man med robust H^ reglering lyckats reducera pitch variationen med 20 % utan att försämra effektregleringen. En grupp från Kassel visade i artikeln "Reduction of fatigue loads on wind energy converters by advanced control methods" att det var möjligt att reducera flapmomentet genom att utgå ifrån en alternativ (stationär) strategi som innebar minskad effekt. Dessutom presenterades idéer om

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aktiv dämpning av tornrörelsen med hjälp av pitchreglering, samt yaw- och tiltkompensering med individuell pitchreglering. Från VKK presenterade T. Ekelund artiklen "Continuos yaw-control for load reduction" som behandlade möjligheten att minska dynamiska laster genom styrning av sidvridsservot. Teoretiska studier visade att lasterna i tornet minskade.

4.6 Nätintegration Avdelningen Grid Integration hade indelats i två sessioner, Grid Integra- tion A och Grid Integration B. Ordförande i den första sessionen, Grid Integration A, var den mycket kunniga David Milborrow. Denna mycket intressanta session inleddes av Gerhard Gerdes som gav en överblick av elkvalitetsmätningar med artikeln "Overview and development of procedures on power quality measurements of wind turbines". Artikeln var för- fattad av G. Gerdes, F. Santjer och R. Klosse, samtliga från DEWI i Tyskland. I Tyskland klassificeras vindkraftverkens elektriska egenska- per. Syftet med denna klassificering är att bestämma vilken påverkan vindkraftverken kommer att ha vid en nätanslutning. DEWI har hittills mätt elkvaliteten på ett 40-tal olika vindkraftverk. Tyvärr "äger" inte DEWI mätningarna, så all publicering av elkvaliteten från ett visst fabri- kat eller en viss typ av vindkraftverk omöjliggörs. I denna presentation hade vindkraftverken indelats i olika grupper med avseende på effekts- torlek, ex 400-600 kW. John Olav Tände tidigare anställd på Ris0 (numera verksam vid EFI) presenterade artikeln "Power quality requirements for grid connected wind turbines". Artikeln som var skriven tillsammans med Paul Gardner från Garrad Hassan and Partners Limited behandlade resultaten från IEC:s arbete i TC 88 med samma titel som artikeln. IEC:s arbete behandlar vindkraftverk anslutna till trefasnät och med en märkeffekt över 50 kVA. Kortfattat kan sägas att arbetet innefattar fyra delar; karaktäristiska elkva- litetsparametrar för vindkraftverk, mätprocedurer, elkvalitetskrav samt metoder för att fastställa vindkraftverkens inverkan på elkvaliteten. I ett EU-projekt har en datormodell för beräkning av flicker från vindkraftverk utvecklats. Detta arbete presenterades i två artiklar, dels artikeln "An electrical system model for predicting flicker from wind turbines", dels artikeln "Design tools for the prediction of flicker". I den senare presenterades av E. A. Bossanyi från Garrad Hassan and Partners Limited och den tidigare presenterade av N. Jenkins från UMIST. Resultaten visar att såväl X/R-förhållandet som kortslutningskvoten på nätet påverkar flickernivån.

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Ordförande i den andra mindre intressanta sessionen, Grid Integration B, var Rich Watson från University College i Dublin. Sessionen innehöll sju presentationer varav tre artiklar behandlade prognostiseringen av vind och därmed förutsägelsen av vilken uteffekt vindkraftverken kan tänkas ge i ett kortare och längre tidsperspektiv. Dessa tre var L. Landberg, Ris0, med artikeln "Predicting the power output from wind farms", G. Kari- niotakis et al med artikeln "Advanced short-term forecasting of wind power production" och T. S. Nielsen et al med artikeln "Statistical methods for predicting wind power". I två artiklar behandlades långsiktig planering av vindkraft i elsystemet. Artiklarna hette "Long term planning of wind energy in conventional power systems" skriven av P. Skjerk Christensen, L. H. Nielsen och K. Skytte samt "Electrical power from widely-dispersed wind turbines" för- fattad av M. Durstewitz et al. De sista två artiklarna "Power control for wind turbines in weak grids" av John Olav Tände och Henrik Bindner samt "Control requirements for optimal operation of large isolated systems with increased wind power pe- netration" av M. Papadopoulos et al behandlade hur modern kraftelektro- nik kan användas för att minimera påverkan från vindkraften i svaga eller autonoma nät. Ola Carlson från Elkraftteknik på Chalmers deltog i en studieresa till nordvästra Irland där besöktes tre vindfarmer. Två med Vestas turbiner och en med USA tillverkade Zond vindkraftverk. Den vindparken med sex Vestasmaskiner hade en medelvind på 10 m/s och var utrustad med en "Voltage control unit". Denna enhet fungerade så att om nätspänning- en blev för hög på grund av låg konsumtion på nätet och hög vind- kraftsproduktion, minskades effekten från ett vindkraftverk tills att nät- spänningen inte blev för hög. Vad som var gemensamt för samtliga upp- ställningsplatser var de stora väganläggningarna i något fall 5 km och 20 % av anläggningskostnaden. Vägarna var byggda så att de "flöt" ovanpå torven som täckte marken i dessa trakter. Vidare berättade personalen från Zond att deras nästa vindkraftverk var ett 700 kW aggregat med variabelt varvtal. Elsystemet för detta aggregat bestod av en släpringad asynkrongenerator med en rotorkaskad av IGBT- omriktare för en mindre del av effekten.

4.7 Buller I det följande ges en sammanfattning av de fyra föredrag och fem posters som presenterades vid konferensen och där innehållet huvudsakligen be- rörde bullerfrågor. I vissa fall presenterades i flera posters eller ett före- drag plus ett poster vilket förklarar varför sammanfattning inte upptar nio rubriker.

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H. Klug et al (DEWI): Noise from wind turbines or: how many Me- gawatts can you get for a 100 dB(A)? Klug visade i sitt föredrag att ett typiskt aggregat på 150 kW för fem år sedan hade en ljudeffektnivå på 100 dB(A). Samma värde gällde för tre år sedan för ett aggregat på 500 kW och nu talar man om aggregat i mega- wattklass med samma ljudemission. Klug påpekade att detta har åstad- kommits tack vare kundernas tryck och med ganska enkla tekniska åtgär- der (dvs huvudsakligen reducerad bladspetshastighet). Mekaniskt buller har också minskat påtagligt, men har fortfarande avgörande betydelse för kringboendes acceptans av aggregatet.

S. Ljunggren (KTH): A new IE A document for the measurement of noise immission from wind turbines at receptor locations. Ett nytt IEA-dokument har just blivit färdigt, se bilagd kopia av föredra- get.

T. Dassan et al (dvs prof Wagner, Stuttgarts tekniska högskola, och olika medarbetare vid NLR, TNO och VUB): Comparison of measured and predicted airfoil self-noise with application to wind turbine noise reduction. Vid prof Wagners institution pågår ett större arbete beträffande predikte- ring av aerodynamiskt buller. Målet är att få en modell där hänsyn tas till samtliga relevanta indata. I det här aktuella projektet utförs mätningar och predikteringar av bakkantsbuller samt buller orsakat av inströmmande turbulens. 13 olika modeller har testats i två av NLRs vindtunnlar. Om resultaten sägs att de till en del bekräftar de teoretiska predikteringarna men också att de ger anledning till en kritisk granskning av de teoretiska modellerna.

K.A. Braun et al: Noise reduction by using serrated trailing edges. Ämnet har ju diskuterats livligt de senaste åren. I det här aktuella projektet har man gjort mätningar på en turbin (1 MW) där yttre delen av ett blad gjorts sågtandformad i bakkanten. Med hjälp av kraftiga riktmikro- fon kunde minskningen i bulleralstring bestämmas. Författarna värderade mätresultatet till ca två dB. Det kunde dock konstateras att minskningen var större vid låga frekvenser än vid höga, vilket i praktiken medför att minskningen är något större än så.

K.A. Braun poängterade att den sågformade bakkanten nog gav ungefär den bullerminskning som förväntades; problemet var bara att bakkanten gav upphov till buller av ett nytt, tidigare obeaktat slag (när luften passe- rar mellan sågtänderna).

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V outsinas et al (T.U. Athen): Numerical modelling of noise emission and propagation. Emissionsmodellen bygger i hög grad på en tidigare semi-empirisk modell publicerad av Brook m fl 1989. Resultaten kan därför inte bli särskilt exakta men kan vara användbara för uppskattningar i tidiga planerings- skeden. Utbredningsmodellen är förmodligen intressantare. Vid höga frekvenser används en strålgångsmodell, som på något sätt modifierats med hänsyn till refraktionen. Vid lägre frekvenser används en direktbestämning med hjälp av vågekvationen. Hänsyn tas enligt uppgift till markytans inverkan, det framgår dock inte vilken impedansmodell som används.

O. Fégeant (KTH): Measurement of noise immission at receptor locations: use of a vertical measurement board to improve signal-to- noise ratios. Med hjälp av en vertikal mätskiva som underlag för mikrofonen kan man framhäva signalen från turbinen bland allt vindbrus. Resultatet har tagits upp i det nya IEA-dokument för immissionsmätningar. En kopia på före- draget bifogas.

K.S. Dahl et al (Ris0): Prediction of noise generated by low speed, separated, unsteady flow over an airfoil. Naver-Stokes ekvationer löses numeriskt i tre steg. Först bestäms de aerodynamiska lasterna med hjälp av ett förenklat förfarande (fluiden förut- sätts vara inkompressibel). Därefter gör man en korrektion så att man får fram en lösning för kompressibel fluid. I tredje steget beräknas slutligen det aerodynamiska bullret med de tidigare bestämda lasterna som drivan- de termer.

4.8 Acceptansfrågor Acceptansfrågor diskuterades i allmänna termer av flera föredragshållare. Man menade att dessa frågor har en hög prioritet men man gav inga di- rekta råd för hur man skall behandla detta i ett bredare perspektiv kopplat till lokalisering. Karin Hammarlund presenterade ett föredrag om vindkraft och acceptansfrågor, se Appendix 13. En designstudie redovisades av Rob van Beek. Långa slanka torn var re- kommendationen, inte speciellt revolutionerande, men ett av alternativen var en fackverksmast. Motivet för att använda ett sådant var att det går att göra dessa slanka i den bemärkelsen att strukturen är genomsläpplig och inte utgör ett solitt visuellt hinder i naturen.

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Endast en muntlig framställning och en poster behandlade människors uppfattningar om vindkraft ur en social synvinkel. Vindkraftens påverkan på omgivningen hanteras alltså fortfarande framförallt som naturveten- skapliga och tekniska effekter. Designfrågorna börjar färgas av land- skapsarkitekternas nyfikenhet på människors upplevelser men även detta område domineras av uppskattade preferenser. Upplevelseprocessen hanteras alltså som renodlat visuell och förövrigt handlar det om bättre teknik för att mäta buller. Från publiken höjdes röster angående hur man skall hantera människors uppfattningar och protester. I detta fallet efterlyses en anpassad etablering av vindkraft som tillvaratar den berörda befolkning- ens intressen. Det krävs mer integrerad forskning som kan hantera bl.a. den subjektiva upplevelsen av ljudet från vindkraftverk.

4.9 Stora maskiner Torsdagen ägnades åt stora maskiner/WEGA II och framtidens maskiner. Havslokalisering och framtida möjligheter med detta presenterades på ett mycket övertygande och professionellt sätt av P. Morthorst. Denna studie är väl värd att läsa som en introduktion till ekonomi och förutsättningar för etablering av vindkraft till havs, se proceedings när dom kommer.

4.10 Övrigt

Något från avslutningen Till sist några punkter som P. J. Jensen från Risö framlade i sitt slutanfö- rande på konferensen. Om vindkraftverken: • högre flexibilitet i strukturen - beräkningstungt • högre drivline flexibilitet - variabelt varvtal, omriktare • högre flexibilitet i regleringen - anpassning av kontrollsystem för uppställningsplats • adaptiv nätanslutning Vad behövs för en stabil utveckling av vindkraften: • ambitiösa mål - 25 % av EU elenergi • marknads incitament - gröna skatter • ökad R & D program

25 FFATN 1997-48

5 Sammanställning av VKK presentationer

VKK var representerade vid konferensen med 12 föredrag och posters. I denna rapport finns en sammanställning av dessa. Dessutom bifogas någ- ra föredrag med anknytning till VKK:s verksamhet. Samtliga föredrag kommer senare att presenteras i proceedings från konferensen.

[I] Hans Bergström, 'Wind Resources in an Arctic Mountain Valley'

[2] Birgitta Källstrand , 'Low Level Jets in The Baltic Sea Area' [3] Stefan Sandström , 'Simulations of The Climatological Wind Field in The Baltic Sea Area Using a Meso-Scale Higher Order Closure Model'

[4] Anders Björck, Jan-Ake Dahlberg, Anders Östman, Hans Ganander, 'Computations of Aerodynamic Damping for Blade Vibrations in Stall' [5] Hans Ganander, Hjalmar Johansson, Christian Hinsch, Henry Seifert, 'Comparison of Load Measurements Between the Large Wind Turbines Aeolus II and Näsudden IP

[6] Holger Söker, Evagelos Morfiadakis, Theodore Kossivas, Anders Östman, 'Footprinting Wind Turbine Fatigue Loads' [7] Anders Grauers, Ola Carlson, Erik Högberg, Per Lundmark, Magnus Johnsson, Sven Svenning, 'Tests and Design Evalution of a 20 kW Direct-Driven Perma- nent Magnet Generator With a Frequency Converter' [8] Jan Svensson, "The Rating of the Voltage Source Inverter in a Hybrid Wind Park With High Power Quality' [9] Åke Larsson, 'Optimal Size of Wind Turbine Transformer' [10] Tommy Ekelund , 'Continous Yaw-Control for Load Reduction' [II] Sten Ljungren, 'A New IEA Document for the Measurement of Noise Immission from Wind Turbines at Receptor Locations' [12] Olivier Fégeant, 'Measurements of Noise Immission from Wind Turbines at Re- ceptor Locations: Use of a Vertical Microphone Board to Improve the Signal-to- Noise Ratio'

[13] Karin Hammarlund, 'The Social Impacts of Wind Power'

[14] Staffan Engström, Göran Dalen, Jan Norling, (Föredrag utanför VKK, men av allmänt intresse) 'Evaluation of the Nordic 1000 Prototype'

27 FFATN 1997-48

EUROPEAN WIND ENERGV CONFERENCE

IB Monday Tuesday Wednesday Thursday times of sessio • 6th October 7th October 8th October 9th October

I Session IV: European and Session IX: Session X: Session XVTt Session XVIII: . [08:30-10:15] {• Arrival & International Programmes Resource Electrical Large Wind Autonomous • Registration Assessment A Components Turbines Systems & Offshore 1 Break Break Break m Session V: Session VI: Session XI: Session XII: Session XIX: Special Session [10.45-13.00] H Session I: Financial, Business Operation & Grid Integration A Resource on Large Wind Turbines & Closing B Opening & Market Issues Performance Assessment B • [10:15-12:30] H Lunch Lunch Farewell Reception cvtfmi cm !13.00-14.00] • Lunch H Session II: Session VII: Session VIII: Session XIII: Session XIV: [14:00-16:00] 1 Wind Energy Planning & Dynamics Aerodynamics A Grid Integration 8 • Issues & Environmental & Control • Achievements Issues

1 Break Break Break 116:00-16:30] H 1 Session ID: Poster Session & Reception Session XV: Session XVI: [16:30-18:30] • National Loading & Aerodynamics B 1 Programmes Fatigue/Standards • & Policies & Certification

• AGMofEWEA Informal music session in evening Conference Post conference tour of windfarms I [18.30] in using wind industry talent in Banquet in Thursday-Saturday in evening • Bedford Hall • Mother Redcaps a nearby pub Trinity College • Conference [21.00] [20.00] I Reception in 1 St Patricks Hall 1 [18.30-20.00]

Figur 1. Översiktligt konferensprogram

28 FFATN 1997-48

Appendix 1

Wind Resources in an Arctic Mountain Valley

Hans Bergström WIND RESOURCES IN AN ARCTIC MOUNTAIN VALLEY

Hans Bergström

Department of Meteorology, Uppsala University, Box 516, S-751 20 Uppsala, Sweden

ABSTRACT Wind measurements during one year on a 36 m high tower located in an Arctic mountain valley in Lapland, Sweden, have been analysed. The results indicate that high mean wind speed climates may be found also in low elevation terrain of the Scandinavian mountain range. The annual mean wind speed at the height 35 m was estimated to be 7.5 m/s. Strong evidence of channelling was found in the wind direction distribution at, which showed two distinct peaks in the direction of the valley.

course also lots of differences, for example as regards the 1 INTRODUCTION valley widths, the occurrence of lakes or dams in the valley bottoms, and the height differences between the valley It is often assumed that due to sheltering effects upon the bottoms and the surrounding mountains. The exact lower elevation terrain, mean wind speed will be higher on importance of all these factors is presently poorly known, mountains and ridges than in valleys. Earlier investigations but it may be expected that mean wind conditions at least in in the Torneträsk area in northern Sweden show, however, some mountain valleys are more favourable than has earlier that also valleys may have quite a high mean wind speed. been assumed. This will also mean that some easier During several experiments on the ice of Lake Torneträsk accessed mountain terrain could be suitable for wind energy it has been shown that due to channelling effects, winds are from a wind resource point of view, not only the higher often very high over the lake itself and at least also over the elevation terrain without roads and often also with a higher land areas close to its western shores, see [1] and [2]. At frequency of icing conditions. Abisko south of the lake, on the other hand, the mean wind speed is usually much lower. Estimates indicate that at 10 m height the mean wind speed over the lake may be as high 2 MEASUREMENTS as around 6 m/s, which is also true at the western shore of Lake Torneträsk, whereas the mean wind speed at Abisko is In the mountain valley at Suorva (18°12'E, 67°32'N) in only about 3 m/s. northern Sweden, people have often reported rather severe wind conditions. For this reason wind measurements began The reason for this somewhat unexpected high mean wind to be taken in November 1995 on a 36 m high tower speed over the central parts of the Torneträsk valley is due located on a small hill close to the Suorva dam. Wind speed to channelling effects of two physical origins. The first one was measured at 4 heights (10 m, 17 m, 25 m and 35 m) is forced channelling and is of most importance when the using Cassella cup anemometers mounted on booms wind direction is more or less along the dominant valleys. reaching about 1.2 m from the tower. Wind direction was When an air flow meets a mountain range with tops and observed at the 10 m level using a wind vane potentiometer valleys, the air is accelerated when it is forced to pass instrument. At the same four heights where the wind speed between the mountain tops. The second one may be called was observed, the temperature was measured using pressure driven channelling or 'gap winds', and is of most ventilated and radiation shielded Pt-500 thermometers. The importance with wind directions more or less perpendicular data logging system was connected to the serial port of a to the valley axis. With such a wind direction, the air PC, and data was stored on the hard disk as 1 min averages. pressure gradient will be along the valley axis, and this pressure gradient will accelerate the air in the direction The Suorva tower site is located on a small peninsula, at a towards the low pressure, up to some hundred metres height rocky hill with almost no vegetation. Its height is about 20 over the valley bottom. For this effect to become important, m above the upstream lake Suorvajaure. Along the lower it is needed that the surface roughness is very small, so that parts of the valley sides, some mountain birch forests are the friction between the air flow and the ground will also be found. The valley as a whole is here rather narrow, only small. This is often the case in arctic areas, as the ground about 3 km between the rather steep fell sides. The during perhaps half of the year is covered with snow or ice. direction of the valley axis is approximately northwest to Also lakes and dams in the valleys will be favourable for southeast. On both sides of the valley the mountains the gap winds to develop as the surface roughness of water typically reach 1200 to 2000 m above sea level. The water is also very low. surfaces of the lakes and dams on the valley bottom are between 415 and 453 m above sea level. The elevation Many valleys of the Scandinavian mountain range are differences are thus about 1000 m, or even more, between oriented more or less as the Torneträsk valley, with their valley bottom and mountain tops, cf. Figure 1. axes from west-northwest to east-southeast. But there are of Fig 1. Map of the area around Suorva, showing the measurement site Suorva.

measurement period. In Figure 2 the measured daily mean wind speed at the 10 m height is plotted.

3.1 Wind statistics The monthly values of mean wind speed are presented in Table 1. The observed annual mean wind speed, 7.7 m/s at 35 m height, is indeed very high to be at an inland location. In fact it is even higher than observed at many of the coastal sites in southern Sweden, where for example the four year average wind speed at Alsvik on Gotland is 7.0 m/s at 36 m height, while at Näsudden the 10 year average at 38 m is 6.9 m/s. Usually wind speed should have a minimum during summer and a maximum during autumn and winter. Such an annual variation is however not clearly seen in the observations at Suorva. Although a minimum is found in July and August, June 1996 was a windy month. During the rest of the year the monthly averages vary rather Fig 2, Daily mean wind speed at Suorva, 10 m level. sporadically between the months.

Table 1. Mean wind speed at Suorva. The annual mean wind profile is plotted in Figure 3. The increase in mean wind speed from 10 m to 35 m is about Height (m) 1.1 m/s on an annual basis, as can also be seen in Table 1. 10 17 25 35 Although the thermal stratification is slightly stable most of January 8.4 9.2 9.7 10.0 the time, the mean profiles show a close to logarithmic February 6.0 6.5 6.7 6.9 variation with height. The curvature of the profile is even March 7.3 7.9 8.2 8.5 slightly of the type that would be expected for unstable April 6.4 7.0 7.4 7.6 stratification. The reason for this behaviour is the small hill May 5.8 6.3 6.6 6.9 where the measurement tower is located. This hill causes a June 6.5 7.1 7.6 7.9 speed up of the wind in a layer close to the ground, leading July 5.2 5.6 6.0 6.2 to some strengthening of the wind up to 10-20 m height and August 4.5 4.8 5.0 5.2 thereby the observed near logarithmic wind profiles as a September 8.0 8.6 8.9 9.1 mean. October 7.5 8.1 8.4 8.6 Assuming a logarithmic wind profile we have November 6.2 6.7 6.9 7.1 December 7.0 7.6 7.8 8.1 t/=-^-In— (1) year 6.57 7.12 7.43 7.67 k z0 where U is the mean wind speed, u. is the friction velocity, k=0.4 is the von Kårmän constant, z is the height above 3 RESULTS ground, and ZQ is the roughness parameter. By regression the observed mean wind profiles could be adapted to the Data from one year, 14 November 1995 to 13 November logarithmic profiles, and the surface roughness parameter 1996, have been analysed from the Suorva site. Due to may be determined. As a mean we get z«=0.01 m with a problems with the data logging system at Suorva, there are standard deviation of 0.01m. This value could be expected some periods with missing data, but as a whole, data is as an overall average in the type of terrain we have at available for 74% of the time during the one year Suorva, with alternating almost bare rocks, low vegetation, some mountain birch, water surfaces, snow and ice cover. 40 (2)

/ 30 where U is the mean wind speed and f is the relative frequency distribution. This distribution is thus completely / described by two parameters, the scale factor A and the form factor c. These two parameters have been determined 20 from the one year wind observations at Suorva, and the resulting theoretical distribution is given by the full line in / Figure 5. The Weibull parameters are also given in Table 2, together with the median wind speed and the maximum 1 1 0 min and 10 min average wind speeds. 8.5 7 7.5 wind speed (m 1% ) The highest wind speed observed during the one year of Fig 3. Annual mean wind speed profile at Suorva. measurements at Suorva is with 1 min averaging time 29.8 m/s and 32.2 m/s at the 10 m and 35 m levels respectively. The corresponding values for 10 min averaging time is 27.0 m/s and 30.3 m/s. This is higher values than has been \y observed during 4 years of measurements at Alsvik on .——^ Gotland in the Baltic Sea, where the maximum 10 min /- average value at 36 m height is 27.4 m/s. Following the extreme value analysis presented in [3], the values 30.3 m/s and 32.2 m/s for 10 min and 1 min averages respectively, -— -^y should correspond to an annual 3 s maximum of about 37 m/s and a 30 year 3 s maximum of about 41 m/s with a 95% confidence limit.

1 0 Table 2. Weibull parameters together with the median tim e (M wind speed and the maximum 1 min and 10 min Fig 4. Annual average of the daily variation of wind average wind speeds. speed at Suorva. Heights 10 m, 17 m, and 35 m. height scale form media max max (m) factor factor n 1 min 10 min (m/s) (m/s) (m/s) (m/s) 10 7.51 1.72 6.07 29.8 27.0 17 8.14 1.75 6.60 31.2 28.6 35 8.75 1.76 7.11 32.2 30.3 The wind direction distribution is shown in Figure 6. As could be expected the channelling effects of the valley are clearly seen with two dominant peaks in the distribution. Two secondary peaks can also be seen in the wind direction distribution. One at about 30° and the other at about 205°. The first one corresponds to winds down the very steep 5 10 1S 20 mountains to the east of the valley, more or less parallel to w ind speed

3.3 Icing conditions Ice on the rotor blades of a wind turbine may be a serious 5 REFERENCES problem in arctic mountainous areas, as for example have been reported from northern Finland. No measurements of [1] Smedman, A.-S. and Bergström, H., 1995: An humidity have been made at Suorva during the year of experimental study of stably stratified flow in the lee of observations analysed here, but at two weather stations in high mountains. Monthly Weather Rev., Vol. 123, No. 8, the area, Ritsem and Vietas, humidity is measured. The 2319-2333. frequency distribution of relative humidity at Ritsem 1981- [2] Smedman, A.-S., Bergström, H., and Högström, U., 1996 show a peak around 80% to 90% relative humidity. 1996: Measured and modelled local wind fields over a Only about 9% of the time the relative humidity exceeds frozen lake in a mountainous area. Contr. Atm. Phys., Vol. 95%. The corresponding distribution for Vietas, using the 69, No. 4, 501-516. limited amount of data available from February 1996 to [3] Bergström, H., 1992: Distribution of extreme wind January 1997, show a relative humidity above 95% just 2- speed. Wind Energy Report WE92:2, Department of 3% of the time. The part of time with a relative humidity Meteorology, Uppsala University. 31 pp. that is high enough for rime, ice, or dew to be formed, is thus relatively small. But for icing conditions to occur, the temperature must be below 0°C. Investigating the ACKNOWLEDGEMENT probability distribution of relative humidity after having applied the condition that the temperature is below 0°C, The measurements at Suorva and the data analysis have show that on an annual basis the part of time when icing been sponsored by Suorvavind HB, Mikael Segerström, conditions may occur is only 4.4% at Ritsem and 1.4% at Jokkmokk, with funding from the regional authorities in Vietas. For individual months icing may as a maximum Luleå. FFATN 1997-48

Appendix 2

Low Level Jets in The Baltic Sea Area

Birgitta Källstrand LOW LEVEL JETS IN THE BALTIC SEA AREA

Birgitta Källstrand

Department of Meteorology, Uppsala University, Box 516, S-751 20 Uppsala, Sweden e-mail: [email protected] ABSTRACT Both measurements and simulations with a higher order closure model indicate that low level jet is a common phenomenon in the Baltic Sea area. LLJ is an important factor for the wind climate, giving a higher mean wind speed. A large amount of the LLJs are supposed to be caused by an inertial oscillation in space when relatively warm air flows out over colder water. A part of the LLJ are caused by a sea breeze circulation. To conclude, it is important to take the effect of the LLJs into consideration calculating the wind energy potential offshore.

1 INTRODUCTION sional, non-hydrostatic model with first order closure. In the comparison by [6], both models gave satisfactory The interest in siting wind turbines offshore is increasing. results, producing LLJ caused by an inertial oscillation. In general a higher mean wind speed and lower turbulence However, differences in details, both between model results intensity can be expected off shore. But, the wind climate in and measurements and between the results of the coastal areas and also further out over sea may show quite large local variations. When the wind blows from sea towards land, the wind speed generally decreases, a decrease that often start a shorter or larger distance upstream the coastline. But, measurements shows that this is 65° « not always the case. During certain conditions, the wind speed may be equally high or even higher close to the coast than further offshore. Two examples when this may happen are when we get a low level jet caused by an inertial oscillation in space and when sea breeze evolves. Low level jets (LLJ), i.e. wind maxima at relatively low levels, have been observed during several experiments at different sites along the Swedish east coast and over the Baltic Sea. For example airborne measurements over the sea during May 29 to June 15 1989, in the vicinity of Ut- längan (A in Figure 1), showed some kind of low level wind speed maximum in 12 of the 25 profiles [1]. During 55 April 30 to 14 June, 1991 pilot balloon trackings were performed at Utlängan, once a day at stochastic hours. The pibal measurements resulted in 52 profiles, corresponding Fig. 1 Map over Scandinavia, with sites mentioned in to 62% of the days. 38 (73%) of the pibal profiles showed a the text marked. A: Utlängan, B: Näsudden, C: more or less distinct LLJ. Figure 2 shows two examples of profiles from these measurements, one profile with and one Östergarnsholm, D: Nässkär. without a LLJ. Also in measurements from Nässkär [2] (D 1000 in Figure 1) and Näsudden (B in Figure 1), LLJ have been 900 ill studied. In fact, during an experiment at Näsudden in 1988, 800 a LLJ was observed in every profile with onshore winds. / I The low level jets observed in this area, or at least a large 700 / h amount of them, are supposed to be caused by an inertial (( oscillation in space when relatively warm air flows out over 500 colder water. 400 \ \ A low level wind maximum caused by an inertial oscilla- 300 i tion, a sea breeze circulation or a thermal wind can not be 200 ( simulated by simple models that often are used to calculate 100 the wind energy potential. Simulation with a meso-scale 10 15 higher order closure model, the MIUU model (see e.g. [3]), Wind speed (m/s) have been performed for situations with LLJ ([2], [4], [5], Fig. 2 Two examples of wind speed profiles, with (full [6],). The results is in good agreement with the measure- line) and without (dashed line) a LLJ. From the pibal ments. [6] also simulated two situations with LLJ over the trackings at Utlängan, 1991. Baltic Sea with the KAMM model, which is a three-dimen- two models, may still be subject to further investigations. ance of 200 km from the coastline. A schematic illustration Simulations with the MIUU model for a large part of the (assuming a wind direction about south-west) is shown in Baltic Sea gave a wind speed maximum at 150 meters Figure 2: Air from a warm land area is transported out over height on an climatological basis [7], [8]. These simulations a cold sea. Frictional decoupling occurs and an inertial also show that low level jets give an increase in wind speed oscillation evolves (I in Figure 2). The wind accelerates of the order of 10% in the Baltic Sea area. during the transport over the water (II). After about 5-7 hours a wind speed maximum has developed (III). If the air reaches a coast, the inertial oscillation is disturbed and the 2 MEASUREMENTS AT ÖSTERGARNSHOLM LLJ disappears (IV). For the case of advection over the Baltic Sea, the development of the inertial oscillation is in The most recent measurements are performed at a very most cases disturbed by changing conditions before a full small, flat island with sea fetch for a large wind direction period of the oscillation is completed. A full period of an sector. The island, Östergarnsholm (D in Figure 1), are inertial oscillation is about 14 hours (at these latitudes) - situated about 5 km east of the larger island Gotland. Three i.e. after 14 hours transport over water the wind speed measurement campaigns have been performed, the first in profile is 'normal' again. spring 1995, the second in autumn 1996, and the third campaign in spring 1997. Continuous measurements of 0 3 6 hours wind, temperature and turbulence were performed on a 30 m high tower at the southernmost tip of the island. During all three measurement campaigns, pibal trackings and radiosonde measurements were performed. The pibal Ill .. ^J&w~ trackings gives profiles of wind speed and wind direction Al Germany and the radiosonde measurements gives profiles of temperature and humidity. In 1995 and 1997 also aircraft measurements were performed in the area, giving wind, temperature and humidity. Tether sonde measurements were performed during the measurement campaigns in 1996 and 1997, giving profiles of wind, temperature and humidity. In pilot balloon measurements performed during Fig. 3 Schematic illustration of a LLJ caused by an May 29 to June 15 1995 at Östergarnsholm, a low level jet inertial oscillation (see text). The vector sum of the is seen in 43 of 51 profiles [9], During the pibal geostrophic wind (dashed line) and the deviation measurements from the period April 30 to June 14, 1997, from the geostrophic wind (dotted line) gives the LLJ occurred in about 2/3 of the 36 profiles. In September 1996, a low level wind speed maximum occurred for a resulting wind (gray full line). longer or shorter period on four of the eight days with pibal As already mentioned, the wind speed profiles from the measurements. measurements at Östergarnsholm in spring 1995 shows a LLJ during large part of the time. As for the experiment at Utlängan in 1991, the land temperature is higher than the 3 LOW LEVEL JET water temperature and most of the LLJ are supposed to be caused by an inertial oscillation in space. An example is Low level jets is a feature that has been observed in many shown in Figure 4. A time series of (a) wind speed and (b) parts of the world and there are several physical processes wind direction for June, 6 are shown, based on profiles which may cause a LLJ. The Baltic Sea jet often develops from the pibal trackings performed during this day. A LLJ, when air is flowing out over the coast. Frictional decoup- with its maximum at about 150 meters height is seen, ling occurs over the sea when the sea surface is colder than increasing in strength from the morning. As clearly seen in the land, and an inertial oscillation evolves in space during the Figure, a large part of the wind speed profile is affected the subsequent transport over the water. The measurements by the LLJ, giving a wind speed increase in a height from Utlängan 1991 have been used to investigate the interval, not only at the height of the maximum. probability for inertial oscillations to be the cause of these LLJs [10]. About 2/3 of the profiles with low level jets In Figure 5, an example of a simulation, with the MIUU fulfilled the examined criteria for an inertial oscillation in model, are shown for a situation with a LLJ caused by an space. Probable reasons for the remaining cases with LLJ inertial oscillation. The geostrophic wind is in this case are mainly sea breeze circulations and the thermal wind. from south-east, so the upwind coast is the Baltic States. The wind speed increases with distance from coast, 3.1 LLJ caused by an inertial oscillation reaching a maximum of about 11 m/s. When the air reaches In the case of a LLJ caused by an inertial oscillation in land (here the island Gotland), the inertial oscillation is disturbed and the LLJ disappears, giving an abrupt wind space, the wind maximum appears at a certain distance speed decrease along the coast. from the coast, because the air is adverted out over the sea. If the air is adverted with a mean wind speed V.ac)v, the wind maximum appears at a distance tmax-Vadv from the coast, where tmax is the time it takes for the inertial oscillation to cause a LLJ, i.e. 5-7 hours (at 55°-60° latitude). With an initial wind speed of, for example, 10 ms"1 and assuming straight trajectories, the wind maximum appears at a dist- speed. The LLJs may give a larger wind shear than may be expected over sea in general. But, this shear is still smaller than the wind shear that in general can be expected over land, so the net effects of the LLJs over sea is positive for the wind energy potential.

1000,

80 100 120 140 160 180 200 220 240 west-east (km) 11 13 15 Time (hours) Fig. 7 Numerical simulation of a situation with a sea (b) breeze circulation. Geostrophic wind: 10 mis, 290°. 100O1

800 6 REFERENCES §, 600 [I] Tjernström, M. and Smedman, A., 1993: The vertical turbulence structure of the coastal marine atmospheric "5 400 boundary layer. J. Geophys. Res., 98, C3,4809-4826. [2] Smedman, A., Bergström, H. and Högström, U., 1995: 200 Spectra, variances and length scales in a marine stable boundary layer dominated by a low level jet. Boundary- 0 11 13 15 Layer Meteorol., 76, 211 -232. Time (hours) [3] Enger, L., 1990: Simulation of dispersion in a Fig. 6 Time series of (a) wind speed and (b) wind moderately complex terrain. Part A. The fluid dynamic direction during an evolution of a sea breeze. Based model, Atmos Environ., 24A, 2431-2446. on pibal and tether sonde measurements performed at [4] Högström, U. and Smedman, A., 1984: The wind Östergarnsholm 4 May, 1997. regime in coastal areas with special reference to results obtained from the Swedish wind energy program, A large amount of the LLJs are supposed to be caused by Boundary-Layer Meteorol., 33, 351-373. an inertial oscillation in space when relatively warm air [5] Bergström, H., 1992: A climatologica! study of wind flows out over colder water. In addition, the criteria for LLJ power potential in the Blekinge area using a meso- y-scale caused by an inertial oscillation to develop are probably higher order closure model. Wind Energy Report WE fulfilled also in other areas around the world. As an 92:01, Dept. of Meteorol., Uppsala Univ., Sweden, 74 pp. example, [11] conclude that LLJ seen in measurements [6] Mohr, M., 1997: Comparisons of Simulations with Two performed in Canada, may be caused by an inertial Meso-scale Models, the MIUU Model and the KAMM Mo- oscillation in space. A part of the LLJ in the Baltic Sea area del, Using Two Low-Level Jet Cases over the Baltic Sea, are caused by a sea breeze circulation. That sea breeze Wind Energy Report WE 97:2, Dept. of Meteorol., Uppsala circulations occur at many sites around the world is well Univ., Sweden, 107 pp. known. But, it is important to be aware of the variability in [7] Sandström, S., 1997: Simulations of the Climatological the wind that may be caused by irregularities in the Wind Field in the Baltic Sea Area Using a Meso-scale coastline, the topography and other factors. Higher Order Closure Model. J. Appl. Meteorol. In press. To conclude, it is important to take the effect of the LLJs [8] Sandström, S., 1997: Simulations of the climatological into consideration calculating the wind energy potential wind field in the Baltic Sea area using a meso-scale higher offshore. Since LLJ is not simulated by simple wind models order closure model. Proc. of EWEC'97, Dublin, 1997. that often are used to calculate the wind energy potential, [9] Smedman, A., Bergström, H. and Högström, U., 1996: this is unfortunately not always the case today. The Turbulence Regime of a Very Stable Marine Airflow with Quasi-frictional Decoupling, J.Geophys. Res. In press Acknowledgements: The author wants to acknowledge Hans [10] Källstrand, B., 1997: Low Level Jets in a Coastal Area Bergström, who has performed the numerical simulations During Spring, to be submitted to Contr. Atmos. Phys and all persons involved in the measurement campaigns. [II] Angevine, W. M., Trainer, M., McKeen, S. T., Berkowitz, C. M., 1996b: Local meteorological features affecting chemical measurements at a North Atlantic coastal site, J. Geophys. Res., 101, D22, 28 935-28 946. Looking at a weather map, a sea breeze situation is 1000 characterized by onshore winds all along the coast of the Baltic Sea and at the same time a uniform wind direction at inland weather stations. Also the wind direction taken from a pibal profile, performed during a sea breeze, changed distinctly at a certain height. Above this height, the wind direction agreed well with the observations from the inland stations. The change could be as much as 180°. The conclusion is that the LLJ seen in the pibal profiles are caused by the sea breeze circulation.

10 11 In the measurement campaign during May 1997, a high Time (hours) frequency of LLJ was seen in the profiles. During this (b) campaign, wind profiles from both pibal and tether sonde ioor measurements are available. In Figure 6, an example of a 80I LLJ caused by a sea breeze situation is shown. The time series of (a) wind speed and (b) wind direction are based on 60( the pilot balloon and tether sonde measurements performed during May, 4. It is clearly seen that the wind direction » 40( changes abruptly, when the sea breeze starts. The wind speed is almost zero at the height where the wind direction 20! changes. The height interval affected by the sea breeze circulation grows during the day, up to about 600 meter. A wind speed maximum appears close to the ground, with a 8 9 10 11 12 Time (hours) maximum of 7 m/s at about 100 m:s height. Fig. 4 Time series of (a) wind speed and (b) wind A simulation, with the MIUU model, of a case with a sea direction during a day with a LLJ caused by an iner- breeze situation is shown in Figure 7. In this example, with tia! oscillation. Based on pi bal measurements per- a quite strong (10 m/s) geostrophic wind from north-east, formed at Östergarnsholm 6 June, 1995. two sea breeze circulation have evolved at the east coast of Gotland. Thus, the wind direction in the sea breeze is

280j ..r almost opposite to the geostrophic wind direction, giving 9 6 26C low wind speeds. In this example, there are large local 9 \ irregularities in the wind pattern. Such irregularities may be 9 9 24C quite common and are caused by numerous factors, for i~ example the shape of the coastline and the topography. 22C /^ 9 > i -\ •^ 20C * 3.3 LLJ caused by other mechanisms o 180 y"l Mm ^ cg A LLJ may also be caused by other mechanisms than an s. / s~ J/ inertial oscillation or a sea breeze circulation. One of these £ 160 CP ( factors is the thermal wind, i.e. the vertical shear of the , X O) S 14C 1 J geostrophic wind. For some of the cases from the investi- 12C CO gation of the data from Utlängan 1991, according to a * )\ r, 1 weather maps and other information, a thermal wind in 10C *'- connection with a front passage was the most probable cause of the LLJ. In contrast to the sea breeze and the 8G <5 20 40 60 80 100 120 140 160 180 200 220 240 inertial oscillation in space, a thermal wind may occur at west-east (km) any time of the year. Fig. 5 Numerical simulation of a situation with an During the measurement campaign at Östergarnsholm in inertial oscillation. Geostrophic wind: 8 m/s, 150°. September 1996, LLJ occurred in a number of wind profiles, even if the frequency seems to be lower than during 3.2 LLJ caused by a sea breeze circulation the measurements performed in spring. A lower frequency A part of the LLJs in the Baltic Sea are caused by a sea of LLJ could be expected, since LLJ caused by inertial breeze circulation. The a temperature difference, between a oscillations and sea breeze circulations are less probable to heated land area and a colder sea do that a sea breeze may appear. However, it is possible to conclude that LLJ of diff- evolve, giving onshore winds. This is a common erent origin thus affect the wind conditions in the Baltic phenomenon mainly during spring and summer. A low Sea area not only in spring and early summer, but for most geostrophic wind speed is favourable for a sea breeze to of the year. develop. A sea breeze may also evolve during higher wind speeds, at least on the leeward' coast according to measurements and simulations. A sea breeze circulation in 5 DISCUSSION AND CONCLUSIONS an area with an offshore geostrophic wind direction may Both measurements and simulations with a higher order thus cause a wind direction change of 180° in the height closure model indicate that low level jet is a common interval affected by the sea breeze. phenomenon in the Baltic Sea area. LLJ is an important factor for the wind climate, giving a higher mean wind FFATN 1997-48

Appendix 3

Simulations of The Climatological Wind Field in The Baltic Sea Area Using a Meso- Scale Higher Order Closure Model

Stefan Sandström SIMULATIONS OF THE CLIMATOLOGICAL WIND FIELD IN THE BALTIC SEA AREA USING A MESO-SCALE HIGHER ORDER CLOSURE MODEL

Stefan Sandström Department of Meteorology, Uppsala University Box 516 S-751 20, Uppsala, Sweden

ABSTRACT A three dimensional meso-scale numerical model is utilised to investigate the climatological wind field over the Baltic Sea. The model results have been compared with measurements from two lighthouses outside the Swedish coast as well as with an interpolation of ship measurements. The comparison shows good agreement between the modelled wind field and measurements, with deviations less than 0.5 ms1 in the main part of the Baltic sea. Thus, the small deviations between measurements and simulations suggests that the main flow forcing parameters, the climatological statistics, and assumptions made are correct.

1. INTRODUCTION 3. MODELLING THE CLIMATOLOGICAL WIND FIELD The major problem concerning wind power is the large number of wind turbines that must be installed in order to In the ideal case climatological study all synoptic and obtain an energy production that can substantially boundary conditions should be covered, but this would contribute to the total energy production and replace other require an unrealistic large number of simulations. Since energy sources. Offshore siting of wind turbines will the MIUU model is rather computer time consuming to run, therefore be more attractive in the future, since the some compromises have to be made. First, the most availability of good land based sites is limited. However, important flow forcing parameters have to be identified. the information about offshore wind resources are sparse. These parameters must then be varied in order to cover a The few existing observing stations are concentrated to the wide range of atmospheric conditions. Parameters coastal areas and only a few offshore. There are important to the wind field are: measurements at a few lighthouses outside the Swedish • geostrophic wind - strength and direction coast. The measurements at the coastal stations are made at • thermal stratification - the daily temperature only one level, usually between 10 and 30 m above sea variation level, while for practical wind energy applications the main • surface roughness interest is focused at higher levels. Consequently, as the • topography data from climatological stations are limited, models are • land-sea temperature differences necessary to obtain a more detailed information regarding the horizontal and vertical variation of the wind field over • thermal wind the sea. The purpose of the simulations presented is to estimate the climatological wind field over the Baltic Sea One simple way to proceed would be to make simulations with a horizontal resolution of about 5 - 10 km and at levels using the mean geostrophic wind speed for a few wind of interest for wind energy. directions; the mean temperature for some months to include the annual differences and a daily variation of the temperature. This approach would however diminish the effect of thermal stratification and also the land-sea 2. THE MODEL temperature differences, and would result in an erroneous climatological wind field compared with observations. A three-dimensional meso-scale model, the MIUU model, Therefore, to include the most important parameters which is used for the simulations has been developed at the affecting the wind field, simulations have been performed Department of Meteorology, Uppsala University, Sweden for the four seasons represented by the climatological [1]. The MIUU model, The model is three-dimensional and temperatures and daily variations for the months January, hydrostatic with a terrain following coordinate system. The April, July and October; 3 geostrophic wind speeds 5, 10, turbulence is parameterized with a 2.5 level scheme. The 15 ms'1 and with eight directions North, Northeast, East, MIUU model has been proved in earlier studies to simulate Southeast, South, Southwest, West and Northwest. Thus we this kind of situations very well when compared with data end up with 96 model runs to cover the most important from several field experiments. parameters determining the boundary layer wind field. All these simulations are then be weighted together using climatological data of the geostrophic wind. The statistics of the geostrophic wind have been evaluated by using a simplified boundary layer wind model described in [2]. With the model it is possible to estimate the geostrophic 90G caused by low level jets, i.e. local wind maxima at Finland relatively low levels [3], [4], [5]. Studies show that the height of the low level jet maximum may vary from less than 50 m and up to 1000 m. Research has shown that there Sweden 9.6 - are many possible causes for the low level wind maximum 700- in the boundary layer. One of these is the warm air advection over a cooler surface. The Baltic Sea is 6oa- surrounded by land surfaces, which means that advective L effects will influence the wind and turbulence structure 500- 9.8 \ regardless of wind direction. The location of the Baltic Sea _ I f at fairly high latitudes causes the land surface temperature 400- to be higher than the sea surface temperature during a large

30a- ;oo 90ft

f 800" Sweden 100f ""-si- Poland 700" 600- 0 100 200 300 400 500 600 700 800 3 500" Distance (km) in 5 Fig 1. Annual mean geostrophic wind speed obtained 400 r from a combination of 1-D simulations and pressure data. 30°)

20O- i. ••.- :£•:• geostrophic mean wind speed was found to vary between be 9.6 ms"1 and 10.2 ms"1 over the Baltic Sea with a standard deviation of 5.7 ms"1. The variation of the geostrophic wind 0 100 200 300 400 500 600 700 800 is shown in Figure 1. Distance (km)

Fig 2. Simulated annual mean wind speed at 66 m. 4. RESULTS

The model results cover the Baltic Sea area and the 90O surrounding countries, an area of 800 km x 900 km. The \: Finland. -^ results are shown in Figures 2 and 3. The modelled wind 80O- ~UrA, o-V^.O speed over land is not shown in the Figures since the Sweden simulations have been focused on the wind field over the 70O- Baltic Sea area. Ä 6001 In Figures 2 and 3, the modelled annual mean wind speed at two heights, 66 m and 153 m are presented. The wind speed gradient at the coasts decreases with height. The ''A • transition from lower wind speed over land to higher wind speed over sea is rather sharp at lower heights while at higher levels it becomes more of a smooth transition zone. As expected, the highest wind speed is found in the central parts of the Baltic sea, with values around 9.5 ms'1 at 66 m and up to 11 ms'1 at 153 m. High wind speeds are also found in the Gulf of Riga, with values about the same as in middle of the Baltic Sea. The annual mean geostrophic wind speed varies spatially between 9.5 and 10.2 ms"1 (Fig. 1) from the north-east to the south-west. The annual geostrophic wind speed is about the same as the wind speed 0 100 200 300 400 500 600 700 800 found at 153 m in the southern part of the Baltic Sea. But in Distance (km) the northern part, the wind speed at 153 m is more than 1 1 ms" higher than the geostrophic wind speed. The high wind Fig 3. Simulated annual mean wind speed at 153 m speed area in the northern part of the Baltic Sea is probably part of the year, which will give a stable internal boundary Table 1. Observed wind speed at Ölands södra grund layer over the sea. At the top of this internal boundary layer and Almagrundet, together with the modelled a low level jet can develop as a result of fractional climatological mean values at same height (about 30 decoupling at the coast [4]. This produces an analogy in m a.s.l.). space to the well-known nocturnal jet [6]. Measurements show that the lowest 100 m of the marine boundary layer over the Baltic Sea is probably stably stratified during 2/3 Almagrundet Ölands södra grand of the year [4]. This indicates that low level jets could be a Month Measured Model Measured Model very often occurring phenomenon over the Baltic Sea. The fact that the annual simulated wind speed at higher levels in Jan 9.4 9.8 9.5 9.6 the Baltic Sea is supergeostrophic, implies that low level jets are frequent and with high speeds. Apr 7.7 7.9 7.8 7.3

5. COMPARISON WITH MEASUREMENTS FROM Jul 6.7 7.1 6.7 7.0 LIGHTHOUSES Oct 8.4 8.5 8.8 8.6 To obtain an estimate of the reliability of the climatological simulations, a comparison with measurements is necessary. Year 8.2 8.3 8.2 8.1 For this purpose, an analysis of the wind data measured at Ölands södra grund and at Almagrundet has been made. Ölands södra grand is situated approximately 40 km away from the Swedish east coast and about 30 km south east of using both wind speed and pressure observations [9]. By the southern tip of the island of Öland. The anemometer is using both pressure and wind speed a better resolution is sited on the helicopter platform at a height of 34 m above achieved, because more information is available. The ship sea level. Measurements from the time periods 1961-70 and observations have also been corrected for coastal influence. 1976-1989 were available, but data for the winter months in Figure 4 shows the difference between modelled wind the second period are very incomplete. Almagrundet is speed at 10 m and the interpolated ship measurements at 10 situated 15 kilometres outside the Stockholm archipelago, m. As can be seen, the agreement is very good, with a which here extends about 20 km east of the Swedish difference less than 0.5 ms'1 in the main part of the Baltic mainland. The anemometer is also here sited on the Sea. The differences that occur might be explained by the helicopter platform and at a height of 31 m above sea level. fact that there are very few ship observations at places Measurements from the time period 1976-78, 1982-95 were where the deviations are largest, that is, the Swedish east available. The anemometers placed at the lighthouse towers coast north of Gotland, the Gulf of Riga and the area are disturbed by the air flow around the tower. Therefore, between Åland and Finland. Another explanation could be correction factors evaluated by SMHI (Swedish the fact that the ship measurements cover only a 4 year Meteorological and Hydrological Institute) based on wind period, 1992-1995, while the simulations are based on a tunnel and full scale measurements at the two lighthouses period of 18 years for the geostrophic wind (1961-1970 and have been applied to the wind data. 1982-1989).

Table 1 shows the observed wind speed at the two lighthouses together with the modelled climatological 7. CONCLUSION values for the months January, April, July and October. Also shown are the corresponding annual mean wind To conclude, it seems possible to use a meso-scale higher- speeds. The agreement between observation and simulated order closure model to resolve the wind field over an area wind speed is very good both at Ölands södra grund and at as large as the Baltic Sea down to horizontal scales of 5 - Almagrundet. The annual measured mean wind speed is 8.2 1 10 km. The simulations presented in this paper and ms" at both Ölands södra grand and Almagrundet. The measurements support the opinion that the increased costs annual simulated wind speed at Ölands södra grund is only associated with offshore siting of wind turbines, can, at 0.1 ms"1 higher than the measured and at Almagrundet the 1 least partly be compensated by the increased energy content simulated wind speed is 0.1 ms" lower than the measured. of the wind. The results can also be used for general siting purposes in the region. But when a particular area has been chosen for wind energy exploration purposes, on site 6. COMPARISON WITH SHIP MEASUREMENTS measurements are recommended.

Figure 4 shows a comparison between the annual modelled wind field at 10 m and ship measurements [8], The ship measurements are from voluntary observing ships in the Baltic Sea during the period 1992-1995 at 00, 06, 12 and 18 UTC. During this period about 20000 ship wind observations were gathered. Most of the ship observations were located in the south-east parts. The synoptic observations from these ships are fitted to a polynom by { Finland Mm

100 200 300 400 500 600 700 800 Distance (km)

Fig 4. The difference between modelled wind speed at 10 m height and an interpolation of ship measurements.

8. REFERENCES

[1] Enger L., 1990: Simulation of dispersion in a moderately complex terrain. Part A. The fluid dynamic model. Atmos Environ., 24A, 2431-2446.

[2] Bergström, H., 1986: A simplified boundary layer wind model for practical applications. J. Climate Appl. Meteor, 25, 813-824.

[3] Källstrand, B.: 1993, Effect of low level jets on wind energy potential in a coastal area. Rep. 93:1. Department of Meteorology, Uppsala Univiversity, Sweden, 54 pp.

[4] Smedman, A., U. Högström, and H. Bergström, 1996a: Low level jets - a decisive factor for offshore wind energy siting in the Baltic Sea. Submitted to Wind Engineering.

[5] Smedman, A., H. Bergström, and U. Högström, 1995: Spectra, variances and length scales in a marine stable boundary layer dominated by a low level jet. Bound.- Layer Meteor., 76, 211-232.

[6] Gerber, H., 1988: Evolution of a marine boundary-layer jet. J. Atmos. Sci., 46, 1312-1326.

[7] Blackadar, A. K., 1957: Boundary layer maxima and their significance for the growth of nocturnal inversions. Bull. Amer. Meteor. Soc, 38, 283-290.

[8] H0jstrup, J., et. al. 1996: Wind resources in the Baltic Sea. Ris0 National Laboratory, Roskilde, Denmark.

[9] Bumke, K. and L. Hasse, 1989: An analysis scheme for determination of true surface winds at sea from ship synoptic wind and pressure observations. Bound. -Layer Meteor., 47, 295-308. FFATN 1997-48

Appendix 4

Computations of Aerodynamic Damping for Blade Vibrations in Stall

Anders Björck, Jan-Åke Dahlberg, Anders Östman, Hans Ganander COMPUTATIONS OF AERODYNAMIC DAMPING FOR BLADE VIBRATIONS IN STALL

Anders Björck*, Jan-Åke Dahlberg*, Anders Östman*, Hans Ganander** *FFA, The Aeronautical Research Institute of Sweden, P.O. Box 11021, SE-161 11 Bromma, Sweden **Teknikgruppen AB, P.O. Box 21, SE-191 21 Sollentuna

Abstract The aerodynamic damping of blade vibrations are calculated. The resulting aerodynamic damping is affected by 1.) the static aerofoil data used, 2.) dynamic stall modelling and 3.) mode shapes. A sensitivity study is carried out and it is found that all three parameters are important. It is shown that the description of the blade oscillation in terms of how the blade deflects is important. Specifically the aerodynamic damping of the lead-lag mode is found to be very sensitive to relatively small out-of plane blade deflections. This means that the orientation of the principal bending axes along the blade is important since this determines the relevant form of blade oscillations. It is thus important to have a correct blade structural description for calculations, but equally interesting is that the aerodynamic damping characteristics can be changed by changing the blade structure.

1. INTRODUCTION mode shapes, which can contain displacement in the flap- The wind turbine industry is today faced with problems wise as well as in the edge-wise direction. For one period regarding stall induced edgewise vibrations. It is of oscillation, the aerodynamic damping work, Wj, is important to identify the causes for such problems and to calculated as develop good design tools. T fVrf = ZI (F,- (f) • tj (t))dt (1) For attached flow and flap-wise vibrations, the / 0 aerodynamic damping of oscillations is normally F/ is the aerodynamic force at blade node / and f,- is the positive. I.e the air extracts energy from the rotor during each cycle of oscillation. For vibrations in the edge-wise speed of blade displacement at node /. The displacement direction, the damping is small or slightly negative. Still, is described by the mode functions as: the positive structural damping is large enough to prevent ri = (Xi,n>Zi) = {fix • ?.9iy • <7>Z}) (2) the blade from developing violent oscillations. with q(t) = qQ sin(O)Qt) When the blade is stalled, the situation is different. The aerodynamic damping is then drastically reduced which The aerodynamic work is normalised to obtain the can result in large vibrations. logarithmic decrement:

The aerodynamic damping is dependent on the type of * = —L O) aerofoils used. The damping is also dependent on the structural behaviour of the rotor and the turbine since the angle of attack history is dependent on the blade motion. where Mg is the generalised mass and qQ is the Further, unsteady profile aerodynamics (dynamic stall) oscillation amplitude of the specific mode at the eigen modifies the static profile behaviour and affects the damping characteristics. Hence, several factors influence frequiency O)Q . the aerodynamic damping. Blade-Element Momentum Theory with a dynamic The current study is performed in order to investigate the inflow extension is used to calculate the inflow relative to influence of various parameters on the aerodynamic the blade. Note that the blade is forced to oscillate at a damping of blade vibrations. prescribed motion. In reality, the forces acting on the blade will result in a different motion. Still, the method Parameters investigated are: can be used to find the sensitivity to different parameters. • Static aerofoil data • Dynamic stall modelling y,flap, out of plane • Blade structure such as mode shapes

2. A MEASURE OF AERODYNAMIC DAMPING Plane of rotation Calculations are made with the blade rotating in x, edge, in-plane homogenous inflow. The blade is also oscillating at its eigenfrequencies, one at the time, with constant Figure 1. Coordinate system showing directions of amplitude. The blade oscillation is described by use of oscillations.

Presented at the European Wind Energy Conference, EWEC'97 Dublin, Ireland, October 1997 3. BLADE USED IN THE STUDY seen that aerofoil v2 is the aerofoil with the most The LM 11 blade used on the DANWIN 180 kW turbine negative lift curve slope for angles of attack in the region is used in the present study. The frequencies of a= 15-20°. In figure 3 it can also be seen that the use of oscillations have been set to 2.45 and 5.37 Hz for the first this aerofoil result in the lowest values of damping of the and second mode respectively. The rotational speed of 1st mode. the rotor has been prescribed to 42.3 RPM. However, for the damping of the 2nd mode the use of 4.FIGURES OF CALCULATED DAMPING aerofoil vl results in more negative damping than using aerofoil v2. It does not seem to exist any such obvious The figures of aerodynamic damping shows the damping indicators as looking at the lift and drag coefficient as the logarithmic decrement in %. curves in order to judge damping characteristics for edgewise vibrations as exist for flap-wise vibrations. The 5. DIFFERENT AEROFOIL DATA slope of the tangential force curve might, however, reveal Different aerofoils will result in different damping something (figure 2b). characteristics. When performing calculations with tabulated aerofoil data it is always a bit of guesswork to 100 define the lift- and drag coefficient curves, especially in the post stall region. In order to investigate how different 80 static aerofoil data influence aerodynamic damping, three 60 O O O -9-^ sets of aerofoil data, vl, v2 and v3, are used in the I current study. 6 40 .\ NVia 20 0.6 I lerodata vl lerodata v2 ...\x-rr 0 terodata v3 -20 10 15 20 25

ö -2 0 10 20 30 Angle of attack o -4 Figure 2a Q (a) and Cd (a) for three different aerofoils -5 10 15 20 25 Wind Angle of attack Figure 3. Damping calculated using three sets of aerofoil 10 20 30 data

6. DYNAMIC STALL It has long been recognised that the inclusion of unsteady aerodynamics (i.e dynamic stall) in aeroelastic codes for wind turbines is essential in order to obtain agreement between measured and predicted loads [1]. The way to include dynamic stall behaviour in aeroelastic codes is commonly to use a semi-empirical model.

Figure 4 shows calculated damping using two semi- empirical models for dynamic stall [2] ,[3]. It is seen that the inclusion of dynamic stall modelling, compared to using steady profile data, results in significant different values of damping in the stall region.

Figure 2b Tangential force coefficient, Ct as function of angle of attack for three different aerofoils.

For the flap-wise damping, the static post stall lift curve slope is somewhat indicative for the judgement of damping characteristics. Looking in figure 2a, it can be 100 100 — quasi-steady profile aerodynamics 80 — FFA.dynanjic stall. 80 . P. . . .Tf=t • Oye dynamic stall, Tf=8 60 0 0 O^} £ — • — TM s 60 t ö ^ .... c 40 g 40 •o 20 2 1 I ° \^ ^*^—.....i' 0

-20 -20 10 15 20 25 10 15 20 25

i=ft=6L

•o — 1 a w •••••jtf> 2 I- •tf \ \ ^^_— •8 3, 1-3 i" \ / -4 V / . . . .V . . - . -5 -5 10 15 20 25 10 15 20 25 Wind Wind Figure 4. Damping calculated using different dynamic Figure 5. Damping calculated using different values of Tf stall models (Tf corresponds to 2*taufac used in [2])

The usefullness of the semi-empirical models is, 7. MODE SHAPES however, dependent on that physical mechanisms of Mode shapes are commonly derived using simplifications dynamic stall are correctly enough described and/or that in such a way that modes have deflections only in pure proper values for semi-empirical constants can be found. in- plane or out of plane motion. For this study, mode shapes have also been derived considering the structural A semi-empirical constant, Tf, is used in both the FFA twist of the blade. Each mode then includes deflections in and Oye dynamic stall models. Tf can be interpreted as a both the in-plane and out of plane directions. time-constant associated with the time-lag of the boundary layer separation process, but it can also be seen Figure 6 shows the calculated blade mode shapes for the as a more general semi-empirical constant that should be 2nd mode1. The in-plane component for the mode tuned to give the best agreement between calculations calculated considering blade twist is very similar to the and experiments. Tuning the dynamic stall model to best in-plane component calcuated neglecting twist, but the fit different 2-D wind tunnel experiments, however, mode now also contains out of plane deflection. Near the results in different best choices for Tf. Such tunings can tip, the blade deflects with an angle of 14 degrees relative typically result in Tf-values in the range between 4 and 8. to the plane of rotation. The deflection is such that when Figure 5 shows that such a variation of Tf results in the blade deflects in the direction of blade rotation it also substantial differences in calculated damping in the stall deflects towards the wind. wind speed range. The reason why different values is needed could be that different values are needed for different airfoils. It is also possible that the model needs < • In plane direction. Mode shape derived neglecting influence or Made twist — In plane direction. Mode shape derived considering blade twist to be refined. Montgomerie has e.g. in [4] suggested that Out of plane direction. Mode shape derived considering blade twisl the time constant should be a function of the angle of attack.

There is, however, a lack of experiments with aerofoils used for large wind turbine blades and experiments for the conditions met at flap- and edge-wise oscillations. Radial position Until such experimental results are available as a base to tune and improve the dynamic stall models, a large Figure 6 Mode shapes uncertainty in the computation of aerodynamic damping will remain due to uncertainties associated with the dynamic stall modelling. The 2nd mode in this context is the 2nd mode counting in frequencies. This mode is sometimes referred to as the 1st edgewise mode using the simplifications mentioned above 100 Figure 7 shows the calculated damping using the mode —©— — Modes un-jjitched shapes derived with and without considering the 80 — Modes pitct ed +3 degrees structural twist of the blade. O O 0 O~iu • • Modes pitched-3 degrees d 100 c 40 x> 6i Modes d "8 80 erived neglegti 1g twist Modes dsrived consider ng twist 20 O O O~Ö^ 60 0 \ o 40 -20 3, V 10 15 20 25 1 o 0 -20 10 15 20 25

—1 -4 6 -5 i- 10 15 20 25 Wind Figure 8. Damping calculated using different pitched -4 modes -5 10 15 20 25 8. CONCLUSIONS Wind The aerodynamic damping has been calculated with Figure 7. Damping calculated using different mode varying dynamic stall modelling, aerofoil data and mode shapes descriptions. It is found that the calculated damping to a large extent is influenced by all these three parameters. The damping for the 2nd mode is seen to be strongly affected by the inclusion of the out of plane deflection. Aerofoil data The out of plane deflection results in a significant In the current study, three sets of aerofoil data have been increase in damping for the 2nd mode. For the 1st mode used. The use of these different data sets as input results the damping is reduced, but the margin to negative in different aerodynamic damping. The variation in damping is still rather high. The derived mode shapes are damping is of the same order as the variation in based on "guesstimates" of the blade stiffness and mass calculated damping due to uncertainties in how to tune distribution. Another guess results in different mode the dynamic stall models. However, the three sets of data shapes and different calculated damping. The figure used does not give the same average power, thrust and shows that the mode shape has a considerable influence root bending moments as a function of the wind speed. If on the aerodynamic damping and it therefore essential to the static data can be tuned to obtain reliable power and have a good description of the blade. root bending curves, then the remaining uncertainty in the computation of aerodynamic damping will be much In order to demonstrate the importance of the inclusion of less. out of plane motion for the damping of the edge-wise mode (2nd mode), calculations have been done with the Dynamic stall modelling modes "pitched". The ratio between in- and out of plane The inclusion of models for dynamic stall has deflection is the same for the whole radius, hence the considerably improved capabilities of proper term "pitched" modes. (Positive pitch means that, for the computations of stall induced vibrations. However, second mode, the blade deflects slightly towards the wind uncertainties in how to tune the dynamic stall models still at the same time as it deflects in the direction of the blade exist. Existing uncertainties in the selection of empirical rotation.). The "aerodynamic pitch" of the blade is the constants result in large differences in calculated same for all cases and the power(wind) curves are damping. In order to tune the models, appropriate wind identical. tunnel tests and unsteady viscous calculations need to be carried out. Results are shown in figure 8. It is seen that a positive pitch increases the damping for the 2nd mode and slightly decreases the damping for the 1st mode. Mode shapes The damping is dependent on the structural behaviour of ACKNOWLEDGEMENTS the blades since the angle of attack history is dependent The work presented in this paper has partly been funded on the blade motion. Specifically the aerodynamic by NUTEK, "National Swedish Board for Industrial and damping of the lead-lag mode is found to be very Technical Development" and partly by the European sensitive to relatively small of out-of plane blade commission within the DGXII JOULE III program under movements. This means that the orientation of the contract JOR3-CT95-0047, Predictions of Dynamic principal bending axes along the blade, which determines Loads and Induced Vibrations in Stall, "STALLVIB". the relevant form of blade oscillations is important. It is thus important to have a correct blade structural REFERENCES description for calculations, but equally interesting is that the aerodynamic damping characteristics can be changed [1] Rasmussen, F. et al., "Response of Stall Regulated by changing the blade structure. Wind Turbines - Stall Induced Vibrations" Ris0-R- 691(EN), ISBN 87-550-1094-8, June 1993. Future work According to the present study the aerodynamic damping, [2] 0ye, S. "Dynamic Stall-Simulated as Time Lag of and hence the development of stall induced vibrations is Separation", IEA 4th symposium on aerodynamics for influenced by the mode shapes. The study is, however, wind turbines. Rome, January 1991, Edited by Ken carried out for the simplified case of a clamped rotating MacAnulty, ETSU England. blade. The next step will be to study the tendency to vibrations for a more complete turbine. The importance [3] Björck, A. "The FFA Dynamic Stall Model. The of couplings between blade motions and motions of the Beddoes-Leishman Dynamic Stall Model Modified for rest of the turbine will be examined. The level of Lead-Lag Oscillations". IEA 10th Symposium on vibrations is dependent on the sum of aerodynamic and Aerodynamics of Wind Turbines i Edingburgh, 16-17 other types of damping. The blade structural damping is December 1996. Editor Maribo Pedersen, DTU certainly important but couplings to the rest of the Danmark turbine will cause motions and possibly extract energy by these movements provided that there is damping in the [4] Montgomerie, B., "Dynamic Stall Model Called system. Studies will also be carried out to identify how "Simple", Rep. No. ECN-C-95-060, Netherlands Energy wind turbulence, yaw, tower blockage etc. influences the Research Foundation ECN, June 1995. tendency to develop stall induced vibrations and large dynamic loads. FFATN 1997-48

Appendix 5

Comparison of Load Measurements Between the Large Wind Turbines Aeolus II and Näsudden II

Christian Hinsch, Henry Seifert, Hans Ganander, Hjalmar Johansson COMPARISON OF LOAD MEASUREMENTS BETWEEN THE LARGE WIND TURBINES AEOLUS II AND NAESUDDENII

C. Hinsch*,H. Seifert*. H. Ganander**, H. Johansson**

* Deutsches Windenergie-Institut, Ebertstrasse 96, D-26382 Wilhelmshaven ** Teknikgruppen AB, P.O. Box 21, S-191 21 Sollentuna

ABSTRACT

This paper is an extract from an extensive report [1] which contains mainly the evaluation and comparison of load measurements and dynamic behaviour for two 3 MW sister wind turbines (AEOLUS II and NAESUDDEN II), which are identical except for the tower, the rotor speed and the control. It also presents results concerning various operational quantities like electrical power, rotor speed and blade pitch angle. Furthermore, the influence of turbulence intensity on loads, accelerations and operational quantities were investigated for AEOLUS II and are shortly presented within the report. The idea of the evaluation is to compare structural behaviour and dynamics of the two turbines at as similar conditions as possible in order to find out the importance of the differences between the two turbines.

I INTRODUCTION signals to the centre line between tower axis and rotor shaft, three translation^ and three rotational accelerations In the framework of the EU funded program "WEGA II were calculated. Large Wind Turbine Scientific Evaluation Project" a sub- project CAN - Comparison of AEOLUS II (located in Ger- many) and NAESUDDEN II (located in Sweden) - was carried out in order to investigate the behaviour of two 3 MW sister wind turbine.s. The two turbines are identical except for the tower, the rotor speed and the control. The AEOLUS II is a two-bladed, variable speed, pitch-controlled 3 MW rated power wind turbine with a hub height of 92 m and a rotor diameter of 80.5 m. The NAESUDDEN II is identically except for the hub height of 78 m and the operation with two fixed rotor speeds. Details about the turbines can be found in the WEGA-II Data Catalogue for Wind Turbines [2].

Fig. 1: Location of the accelerometers in the nacelle. 2. DATA ACQUISITION SYSTEMS AND EVALUA- TION SOFTWARE The operational quantities like rotor speed, blade pitch angle, rotor azimuth angle, nacelle azimuth angle and yaw 2.1 The AEOLUS II status are measured at the turbine's nacelle and transferred to the ground station. The electrical power is measured by a One blade of the AEOLUS II wind turbine is installed with power transducer at the ground station. strain gauges in six full bridges for the measurement of the The wind speed is measured with calibrated cup anemome- loads (3 forces and 3 moments), identically to the NAE- ters at three heights (62m, 92m, 126m). The other meteoro- SUDDEN II blades. All strain bridges are placed on the in- logical quantities are described in [3] & [4]. ner side of the blade root. Because of the short distance to DEWl's philosophy of evaluation software principles takes the blade bearings, non-linearities occur which makes it into account the recommendations given by ELSAM [5] necessary to perform a special calibration procedure. Tak- and the IEA [6] and is based on the aim to save all relevant ing into account all forces and moments as well as the rotor information, without generating hundreds of Megabytes of speed and the blade pitch angle, the edgewise and flapwise data. Therefore, a capture matrix was developed which al- moments are calculated and transformed to the hub-fixed lows to save a given amount of 10-minutes-time series in coordinate system. each matrix element. The matrix is mainly divided into To study the turbine behaviour in general and in special wind speed and turbulence intensity classes of 2 m/s, resp. situations, six acceleration sensors were installed at the 5% intervals. Furthermore, some special matrix fields are turbine's nacelle, measuring the accelerations twice in all reserved for special operational conditions like start/stop at three directions (see Fig. 1). By transforming the single low/high wind speed, high yaw misalignment, high vertical wind shear and yawing. A maximum of ten 10-minutes time series of raw data are stored per matrix element. Dur- ing the whole measurement period, time series were re- Tab. 1: Operational Statistics corded for wind speeds up to 24 m/s and turbulence inten- I AEOLUS II NAESUDDEN II sity levels higher than 20 %. In addition to the time series of raw data (recorded with a energy production 17.719 MWh 17.816 MWh sample rate of 20 Hz), a special file is written during the production 61.3 % 45.5 % whole measurement campaign, containing ten minutes av- stand-by 15.7% 19.2% erages, standard deviation and extreme values of selected fault / maintenance 21.4/1.6% 33.4% quantities. The software flow diagram for evaluation can be start / stop — 1.9% seen in [7]. 4.2 Main Loads 2.2 The NAESUDDEN II

The mean loads of in-rotor-plane moment Min.p|Me and out- The measurement system consists of the distributed data of-rotor-plane moment Mout.of..p|)ine as well as for the flap- acquisition system, the net work, the computer and the wise loading FlapM and edgewise loading EdgeM of the power backup device. When the turbine is running the rotor blade are presented. The shown values of 10-min av- sampling frequency is 20 Hz, otherwise the measurements erages and standard deviation include also periods of yaw- are carried out with 1 Hz continuously. Measured data are ing and all recorded turbulence intensity levels. stored on video8 tapes with capacity 2.5 Gbyte or 5 Gbyte. The bin-averaged curves of Min.p)ai)e (Fig. 2) increase with The data acquisition system is built up by four modules lo- increasing wind speed up to around 14 m/s, where the cated in the rotor system, in the nacelle, at the tower top power reaches its rated value of 3 MW. At higher wind and in the measurement building. Connected to the com- speeds, the blade pitch control reduces further increase, puter there is a monitor which can be used for graphic pres- giving nearly constant power output and constant moment entation of signals, which also can be documented via the Mjn-piwe- The values of average Min.pIåne for NAESUDDEN laser printer. II are at rotor speeds of 21 rpm lower than for AEOLUS II; Two blades are equipped with strain gauges at the NAE- because of the higher rotor speed the driving mean loads SUDDEN II, but only blade 1 is used for the comparison. are reduced at equal power output An important part of using the continuous measurements is

the overview plotting that is done routinely. On one hand —«—MOlUS * 1 - , - «*«i«y»M • 1 they are used to check all signals, on the other hand they J quickly give a picture of wind as well as turbine situations that are measured. Finally, they are complete and contain everything that has occurred, even very unforeseen situa-

tions, and can be used for evaluation of fatigue damage of 4 all situations which have occurred.

3. CALIBRATION PROCEDURES J .-

ft ft 18 1$ » Because of non-linearities due to the short distance of the wind speed v at hub height/ nVt strain gauges to the pitch bearing and temperature effects a

special procedure has been developed in order to calibrate Fig. 2: Comparison of average Min.plme the strain signals at the blade roots. The procedure takes into account rotational speed as well as forces and moments The standard deviation of Min_pluie (Fig. 3) is mainly driven for deriving the resulting edgewise and flapwise bending by the mass of the rotor blade (1175 kNm) with values be- moments [8]. tween 750 and 800 kNm.

4. RESULTS v 4.1 Operational Statistics •

m The energy production of both turbines since starting operation (AEOLUS II: Dec. 1993, NAESUDDEN II: Mar. \ 1993) up to the 31" of August 1997 is given in Tab. 1, to- \ gether with the distribution of operation modes. Both turbines had stand-still periods of around half a year due to . ..HAESUOOCNM gearbox repair in 1996 (AEOLUS II) and 1997 I 1» II (NAESUDDEN II). wind »pnd v « hub halgM I m/i

Fig. 3: Comparison ofstd. dev. o/Min.piane Due to constant rotor speed the variation of loads are lower not differ very much from the values for M^^ and Mout. for the NAESUDDEN II. The course of the curve at low of-pi™. respectively. wind speeds might be influenced by calibration problems For the flapwise and edgewise bending moment of the rotor and therefore needs to be more investigated. blade the 1-Hz-Equivalent Load L^, and a fatigue spectra For AEOLUS II, the bin-averaged curve ofM^^,,,, (Fig. for a given wind speed distribution and assumed wind tur- 4) shows a similar course as the M,,,^^ over the wind bine duty cycles [5] can be determined. speed: Increasing values up to a wind speed of 14 m/s and Due to bad calibration conditions the calibration procedure then a drop of nearly 750 kNm up to 20 m/s. For NAE- for the blade root loads is not possible for low rotor speed SUDDEN II, the average Mcn-of.)»™ has two Ieve's for the (e.g. during start and stop procedures) and the resulting two rotor speeds. At 14 rpm, the loads for NAESUDDEN II values for the moments can not be trusted in these cases. are remarkably lower than for AEOLUS II, which is due to Therefore, start and stop cycles were not taken into ac- lower rotor speed. At 21 rpm, the loads for NAESUDDEN count. No attention is paid to changes between 10-minutes II are higher caused by higher rotor speed compared to periods nor to the changes of rotor speed in the case of AEOLUS II. NAESUDDEN II. Besides the 3-dimensional way to describe the results in -«—UOUJSI - • > *w»ueocN • terms of matrix mean value, rms value and irregularity, the 1-dimensional quantity I-Hz-Equivalent Load L^ is very suitable to describe and compare fatigue phenomena [9], even if no structural dimensions are involved in L^ and thus nothing about the damage level is said. For AEOLUS II, the edgewise 1-Hz-Equivalent Load (Fig. 6) shows an increase with increasing wind speed and a remarkable peak at 5m/s, eventually caused by resonance due to the low rotor speed of around 13 rpm (2p = 0.43 Hz), which is very close to the natural frequency of the tower (0.42 Hz), see the resonance diagram (Fig. 13). For NAE- d » «t hub httght I nV« SUDDEN II, the edgewise 1-Hz-Equivalent Load is lower

Fig. 4: Comparison of average Mm,_of.p)a at wind speeds up to 12 m/s, but then variations in wind speed causes higher equivalent load because of the fixed For AEOLUS II, the standard deviation of M^a^u» (Fig. rotor speed not allowing these variations to be compensated 5) increases slightly from cut-in wind speed up to 13 m/s by variations in rotor speed. and then more significantly, with the blade pitch mechanism in operation. For NAESUDDEN II, the constant rotor speed of 21 rpm also during blade pitch operation leads to 1100 lower load variations. At low wind speeds (up to 9 m/s), the 5 variations in rotor speed are remarkably higher at AEOLUS 1700 fT-'* """'"'" ' II, thus leading to higher variations in M,,,^,,, in this / wind speed range. At higher wind speeds, the variations in /• rotor speed as well as in blade pitch angle,are higher at AEOLUS II. So it can be. concluded that the control of AEOLUS II allows higher variations in operational parameters and leading therefore to higher variations in -*-AflXUSfl . • . NACSUDDCN H Mout-of-p!»™- Perhaps these differences are also influenced by 1000. ^ , , . i ^__ , . the different tower concepts (soft for AEOLUS II, stiff for wind ipMd v «t hub teigttt (12m)' nV» NAESUDDEN II). Fig. 6: Comparison of edgewise equivalent load —_AKKU»I t * «. NUSUOMNN I For calculation of the one-year fatigue spectrum a Rayleigh-distribution (Vro = 8.5 m/s, t; = 0.1-0.15, turbine availability 100%) were assumed. The resulting fatigue spectra (see Fig. 7) shows a typical vertical section repre-

/ senting nearly twice the amplitude caused by the mass of X MO •5 the blade for the operational number of around 7.500.000 cycles. At both turbines, the maximum range of 4300 kNm L occurs in the wind speed class 16-18 m/s. For AEOLUS II, the flapwise 1-Hz-Equivalent Load (Fig. 8) shows also a slight peak at 5 m/s, a slight increase with d » «thub halght; further increasing wind speed and a stronger increase above 13 m/s. For NAESUDDEN II, the increase is stronger at Fig. 5: Comparison ofstd. dev. ofMmhoj:plmt low wind speeds and slighter at higher wind speeds. Over the whole wind speed range, the equivalent load for NAE- The course of the bin-averaged curves for average and SUDDEN II is lower than for AEOLUS II. standard deviation of the moments M^g^ and MMnap) do j takes place, the second increase starts at a wind speed of ! XJUS1 I I UUOMHlf around 11 m/s. The highest translational acceleration is Ay Vr (Fig. 10), representing the deviation of the tower in y-di-

e 310* •••• ! rection, which corresponds to the lateral direction of the ro- •-v - HM tor. The highest rotational direction is Rx, which corre-

2 2IM sponds to the torsional movement (yaw motion) of the tower around the tower axis. f KM

2 1MO ^«A£OU»t 1 . ••NACCUDOeMNrl *M m J 1.Wl«W 1.WE-O1 1«E<« 1MM» >»WI l.«t«l number of cyclM / F/g. 7; Comparison of edgewise fatigue spectra / » u For AEOLUS II, the resulting fatigue spectra (Fig. 9) has a y maximum of 4300 kNm for the wind speed class between 16 m/s and 18 m/s. The values for NAESUDDEN II are lower over the whole load range, having a maximum of • I 10 U » 3700 kNm. wind iDMd v «t hub h*Hih! I mfe

. tlWIIt. Fig. 10: Comparison of std. dev. of acceleration Ay Regarding the influence of the much more soft AEOLUS II tower compared to the stiffNAESUDDEN II tower, the std. I dev. of accelerations within 10 minutes are larger by a factor of 4 at AEOLUS II for wind speeds exceeding 8 m/s. •v V The picture is somewhat more clear when looking at the frequency content as shown in FFT diagrams [1]. The * NAESUDDEN II is more dynamically excited at lower rotor speed than AEOLUS II. The opposite can be seen for the higher rotor speed. However, what is of interest is the I 1» II corresponding loading of tower and nacelle. The relation of wind >pwd v at hub bright (12m) / Wi this loading F between the two turbines can roughly be estimated to be: FAEQLUS J, / FNAESUDDEN u S 0.1. Fig. 8: Comparison offlapwise equivalent load The main reason for this is the low stiffness constant of the 19» AEOLUS II tower.

4SM -4- j. .. NA£«UOCM»lt~ MM -^- 4.4 Comparison of Special Situations •v, The std. dev. within 10-minutes describe more the varia- J IK» tions of quantities with wind speed variations. The short \ term characteristics will be investigated by means of time I"00 5 1K0 series, FFT-Analyses and azimuthal binned curves. '"SJ The AEOLUS II is triggered at the low rotor speed in thrust direction (2p « natural frequency of the tower). The" lp in electrical power indicates that there are some differences ** 1.C0E*« 1.ME-O1 1.ME-K1 U0E

I» NJLtUOOtMI iteo : K.V. .***** '. 2»e A ••' r [ 1HO AT \ 11» • IT» \ * j V s SS -j. . . MACSUDOCN i|-

I• IU M 131 1H 231 370 Ml »0 rotor azimuth ingta / d*g tx u rotor tpwd / Hi

Fig. 11: Azimuthal binned curve ofMouMf.p}ant Fig. 13: Resonance Diagram at high rotor speed As can be seen, for AEOLUS II the natural frequency of For NAESUDDEN II, the electrical power has obvious 2p the tower is very close to the common appearing 2p-excit- at 14 rpm and 20 rpm. The 4p at 20 rpm is also coupled ing frequency in the low rotor speed range. with the tower (acceleration Az). 9p is a very apparent fre- For NAESUDDEN II, the natural frequency of the tower is quency component at the lower rpm, due to differences be- close to the 5p-exciting frequency of the turbine. tween the blades. The rotational accelerations are lower at 20 rpm than at 14 rpm. In comparison, the tower-yaw frequency, which can be 5. CONCLUSIONS seen in the accelerations Ay and Rx, is lower at AEOLUS II (1.46 Hz) than at NAESUDDENII (2.07 Hz). The tower The two 3 MW turbines AEOLUS II and NAESUDDEN II of the AEOLUS II is excited by 2p at the lower rotor speed, are similar except for two major differences: Rotor speed whereas the tower of the NAESUDDEN II is excited by 4p control and tower stiffness, and the question is to what ex- at the higher rpm (Fig. 12). tent divergences of loading and dynamics are influenced by these turbine inequalities. 0.4 1 A^ Regarding the electrical power as an 10-minutes-average, •J f ""Vs. til there seems to be no significant differences between the two turbines, see also [3]. But details during shorter time Ultl 1 SUDOCMlf periods reveals large differences in terms of amplitude" frequency distribution. The AEOLUS II has a much more

* smooth behaviour, which of course reduces load variations '. in the rest of the turbine. On the other hand, which is very • •, 1 t .-•' pronounced at low wind speeds, the variation of rotor speed *•» * ! • • t * causes out-of-plane moment variations during 10 minutes '• • »• nt IM as as an example. The sensitivity of the rotor speed on out-of- retar axfcnutti angle I dag plane moment is quite obvious looking at the NAESUD- DEN II turbine, when changing rotor speed. Fig. 12: Azimuthal binned curve of acceleration Az Regarding the influence of the much more soft AEOLUS II tower compared to the stiff NAESUDDEN II tower, the As can be seen in the azimuthal binned curve of the out-of- standard deviations of accelerations within 10 minutes are plane moment (Fig. II), the blade passage through the larger by a factor of 4 at AEOLUS II for wind speeds ex- tower shadow is not less severe at the AEOLUS II, even if ceeding 8 m/s. The picture is somewhat more clear when the distance between blade and tower is higher because of looking at the frequency content as shown in FFT dia- the slender construction. grams. The NAESUDDEN II is more dynamically excited Finally, it can be assumed that both turbines have blades at lower rotor speed than AEOLUS II. The opposite can be which among themselves are not totally equal. seen for the higher rotor speed. Another consequence of the different towers is that the dis- 4.5 Resonance Diagram tance between the tower and blade during the blade passage is larger at the AEOLUS II. However, this can not be seen By means of a Fast-Fourier-Transformation Analysis the in a reduction of the flap load variations. natural frequencies of the components lower and rotor Regarding evaluation of load variations in terms of fatigue, blade were determined. The following figure shows the the determination of the total spectrum contains only ordi- resonance diagram, containing the measured natural fre- nary situations and for this reason has a relatively limited quencies as well as the excitation frequencies caused by the importance for interpretation of fatigue. Furthermore, load rotor and the operational range. spectrum itself, or the equivalent load, is independent on dimensions and the strains or stresses of the material. However, the project has contributed to lots of measured data and knowledge about behaviour of the two turbines in terms of mean values as well as dynamic response proper- [8] J.Å.Dahlberg, H.Johansson : Calibration Procedures for ties. Improved Accuracy of Wind Turbine Blade Load Meas- urements : in Proceedings of the European Union Wind Energy Conference: Göteborg, 1996. 6. FUTURE WORK [9] H.Söker (Editor): Monitoring Fatigue Loads on Wind Turbines Using Cycle Counting Data Acquisition Systems: The comprehensive measurements are a good basis for fur- Final Report: JOU2-CT92-0175 : DEWI, FFA, CRES : ther investigations concerning the operational quantities, 1995. blade loads and accelerations at AEOLUS II and NAE- SUDDEN II. Nevertheless, more should be done to refine the calibration Acknowled gement procedure by installing also strain gauges on the second blade at AEOLUS II to get a more complete picture of the The work presented in this paper has been co-funded by the loading. The strain gauges then have to be located on the European Commission within the DGXII Non-nuclear En- outside of the blades at a position which is not effected by ergy Programme as a part of the WEGA II project (JOU2- load introductions. Especially, the start and stop opera- CT93-0349). Part of the Swedish work is also funded by tional modes should be modelled more adequate to take the utility company Vattenfall AB and The Swedish Board these situations into account for calculating a life-time fa- of Technical Development (NUTEK). tigue spectra. Also the question of temperature dependency The authors would like to express their thanks to Mr. Bi- is not answered completely. bricher, Mr. Roer and Mr. Wendt from the manufacturer The influence of various further external meteorological MBB and Mr. Göransson from the Swedish manufacturer parameters (e.g. wind shear, wind direction fluctuations, Kvaerner for assistance in the measurements. extremes in terms of wind conditions like gusts) on large DEWI also wants to thank the utility PreussenElektra Han- wind turbines should be investigated and conclusions from nover, here especially Mr. Klemm, and Mr. Hoelscher with the turbulence intensity investigations should be made in his colleagues from the Wilhelmshaven power station, for terms of finding a description of loading as a function of making their turbine AEOLUS II available for the meas- turbulence intensity (e.g. by applying a multivariante re- urements and assisting during the application of sensors, gression analysis or other mathematical procedures). maintenance of the data acquisition systems and during From the knowledge gathered by this project investigations special measurement campaigns in operating the turbine. should be carried out to improve the turbine control Teknikgruppen wants to thank the utility Vattenfall AB for mechanism. their general support of this project and also many thanks In addition more effort should be paid to investigate the in- to Mr. Andersson and Mr. Ohlsson at the NAESUDDEN II fluence of low cycle fatigue on the total fatigue load spec- site for the engagement in running and maintaining the trum, especially when generating a spectrum from 10-min- measurement system. utes time series. Furthermore all turbine conditions should be taken into account and attention should be paid to the differences between 10-minutes situations.

7. REFERENCES

[1] CHinsch, T.Kramkowski, H.Seifert, H. Ganander, H.Johansson, RXindström : CAN Final Report Part 3.3 Load Measurements AEOLUS II - NAESUDDEN II: DEWI, Teknikgruppen AB : 1997. [2] WEGA-II Large Wind Turbine Scientific Evaluation Project: Data Catalogue for Wind Turbines : Elsamprojekt A/S : 1997. [3] A.Albers, C.Hinsch, H.Klug, D.Westermann, G.Ronsten" CAN Final Report Part 3.1 Power Perform- ance: DEWT, FFA: 1997. [4] A.AIbers, C.Hinsch, H.Klug, D.Westermann : Power Performance and Operational Chararcteristics of the AEO- LUS II: Appendix to CAN Final Report : DEWI: 1997. [5] WEGA-II Large Wind Turbine Scientific Evaluation Project : Guidelines and Recommendations : CEC DGXII Contract JOU2-CT93-0349 : Elsamprojekt A/S, ETA Plan: 1996. [6] IEA : Recommended Practices on Wind Turbine Test- ing : No. 3 : Fatigue Loads : 2nd Edition : 1990. [7] C.Hinsch, T.Kramkowski, H.Seifert : Operational Quantities, Blade Loads and Dynamic Behaviour of the AEOLUS II: DEWI: 1997. FFATN 1997-48

Appendix 6

Footprinting Wind Turbine Fatigue Loads

Holger Söker, Evagelos Morfiadokis, Theodore Kossivas, Anders Östman FOOTPRINTING WIND TURBINE FATIGUE LOADS H. Söker*, E. Morfiadakis**, T. Kossivas**, A. östman*** *Deutsches Windenergie-Institut, DEW], Ebertstrasse 96, D-26382 Wilhehnshaven, Germany ** Centre for Renewable Energie Sources, CRES, }9ih km Marathonos Ave., GR-190 09Pikermi Atliki, Greece *** The Aeronautical Research Institute of Sweden, FFA, P.O. Box 11021, S-161 11 Bromma, Sweden

ABSTRACT

The effect of wind farm and complex terrain operation on wind turbine fatigue loading is of great interest but still not easily quantified. Within the EU Non Nuclear Energy R&D Programme the described project Measuring Footprints of Wind Turbine Fatigue Loads Using Monitoring Methods applies a monitoring method on three wind turbines of the same type operating under flat terrain / stand alone, wind farm and complex terrain conditions. Statistics - footprints - of the load quantities are established through on-line rainflow counting of the sampled data. These footprints are evaluated to identify relevant quantities that can serve as shape, intensity and validity parameters. The paper presents the project's objectives and technical approach as well as first measurements and evaluation results.

1 INTRODUCTION Wind turbines have grown large in size and have strongly 2. TECHNICAL APPROACH been modified in key components and employed The approach is to apply a common methodology on all technologies giving rise to a renewed necessity of load test sites in order to create test data of comparable quality measurements. Cost effective, reliable load measurement that can be evaluated. Missing information is to be and load prediction procedures maintaining reasonable complemented by employing FFA's vast data base of safety levels are demanded by industry. measurements at Alsvik and, moreover, by using their simulation capabilities. In acknowledgement of this demand and in the framework of the EU's Non Nuclear Energy R&D Programme a CRES, TACKE and DEWI have jointly selected the wind project consortium established by CRES, FFA, TACKE turbine type TACKE TW 500 - this type of turbine is and DEWI set out to investigate wind turbine fatigue loads available in both countries - and the appropriate test sites within the described project Measuring Footprints of Wind in Greece and Germany. Since there exist two models of Turbine Fatigue Loads Using Monitoring Methods. The the TW 500 machine with slighly different technical data, project employs the fatigue load monitoring method [1] for both sets of characteristic values are given in table 1 along its appealing simplicity. As test turbines three TACKE 500 with those of the turbine type installed at Alsvik: kW machines have been selected, each in their specific external conditions, i.e. stand alone, wind farm & complex TWS00 TW500 Dan win terrain conditions. model I model II 180kW rated power 500 kW 500 kW 180kW The method delivers fatigue load statistics -footprints - diameter 36 m 37 m 23 m through on-line rainflow counting (RFC) [2] of the rpm 29.6 30.9 42 sampled data. A footprint describes the fatigue loading of a blades 3, LM17 3,LM17 3 wind turbine component in terms of occurences of load hub type Y-type spherical, Y-type, cycles for a given measurement time interval, typically one extenders day, 10 days or one month. Such load cycle frequency extenders distributions are to be characterized by shape, intensity upwind, upwind, stall upwind, stall and validity parameters. Each footprint must be associated stall with a set of parameters related to the prevaling Tab. 1: Turbine characteristics operational and external conditions during that time interval. The project work focusses on a proper choice of these governing parameters in a way to enable 2.1 Site Descriptions measurements to be scaled, extrapolated and rated for Toplou site on Crete is characterised by complex validity. Having accomplished this it will be possible to topography. On the greater area, the Public Power investigate the differences in loading of a wind turbine Corporation (PP6-) operates a wind park consisting of operating at different conditions (i.e flat and complex seventeen 300 kW wind turbines and three 500 kW wind terrain or wind farms). turbines, two of which are TACKE TW 500. All of them with the exception of one 300 kW machine are placed in a The potential of the employed monitoring method to assess row with SW-NE orientation. The surrounding area is long periods of operation enables to estimate the relevance predominantly mountainous with many hills and ridges of of low cycle fatigue which has been seen to become various slopes covered with sparse and low lying relevant with the use of modern fatigue resistant materials vegetation of typical Mediterranean nature. The roughness as a consequence of the turbines' growing sizes. length of the area is estimated as 0.10 m in terms of visual inspection. In Germany two suitable model II turbines have been possibility to compose a comprehensive fatigue load found assisted by TACKE: one in the commercially footprint from a pool of 1-day-footprints. operated wind farm of the local utility of the city of FFA has a large data base available from former Emden, SWE, and one privat owned stand alone turbine in measurement campaigns in the Alsvik wind farm. the vicinity of Wilhelmshaven. Both sites feature flat Measurements of blade and tower loads in one of the terrain with meadows, single groups of trees and sparsly turbines, power from all turbines and meteorological data, distributed buildings. The SWE wind farm consists of 10 wind speed and wind direction from 7 levels in two masts, wind turbines organized in two rows of 5 turbines in NW- have been archived. The location of the turbines enables SE orientation. Although the two lest sites are some 70 km investigation of wind farm operation effects for varying apart from each other, the meteorological conditions in the turbine distances. whole of Germany's coastal area are considered to be similar enough as to allow for long term comparison of In order to complement measurements FFA has, assisted measured load footprints. by TACKE Windtechnik, further set up a simulation model in order to perform load calculations for the TACKE 500 FFA has performed measurements in the small wind farm kW wind turbine at Alsvik during many years. The farm consists of four three bladed 180 kW Danwin turbines and the site features 3. SCIENTIFIC APPROACH very flat terrain. The layout of the farm has been suited for wake studies [3]. The principle of footprints refers to the archaic ability of 2.2 Measuring Campaign men to read the traces left in the ground by other creatures and to interprete the information contained in them. Extensive measurement programs are being carried out on Transferring this principle to the task of describing a wind the three TACKE TW500 wind turbines comprising turbine's fatigue loading & footprint is to be understood as meteorological, operational as well as load quantities. a suitable statistic representation of a wind turbine's load All three test turbines have been equipped with strain history under representative external conditions. The gauge bridges for measuring blade root flapwise and central aspect of the project work is to develop the skills edgewise bending moments, tower bending moments and and the experience in reading the information that is tower torsion moments. comprised in such a footprint. Wind speed measurements on the machine operating at 3.1 Fatigue Loading Representation Toplou site are performed using cup anemometers and The fatigue loading of the wind turbine components is vane resolvers at three different heights. In addition, for described using RFC statistics in three representations: the description of the 3-dimesional character of the • Equivalent Load Range, Leq: incoming wind flow, a fast responding sonic anemometer The information contained in the RFC statistics is is used. Meteorological measurements include atmospheric used to calculate the 1-Hz-equivalent-load using the temperature and pressure, while the operational wind Palmgreen-Miner's rule (5]. turbine quantities include nacelle direction as well as • RFC Amplitude spectrum (1-dimensional): active power and grid/brake status. All analogue signals The information contained in the RFC statistics is from the sensors are sent to interface boxes housing power transformed into a (cumulative) frequnency supply modules and transient protectors. After low-pass distribution of load cycle amplitudes ommitting the filtering and A/D conversion a PC is used to control mean value at which the load cycle occurs. automated data acquisition and data storage. Data are continuously recorded as time series on which off-line • RFC Transition Matrix (2-dimensional): processing procedures are applied to obtain results The transition or load cycle matrix contains the equivalent to real monitoring data i.e. on-line rainflow number of occurences of a class of load cycles, counting of the fatigue load. information about their amplitude and their mean value. On the two test turbines in Germany wind speeds are 3.2 Description of The External Conditions measured using the wind turbine's nacelle anemometers with an appropriate calibration [4]. The nacelle wind vane In order to identify and quantify the effects of the external signal can be used to determine duration of oblique inflow. conditions on the machine fatigue loading, the following Again operational wind turbine quantities include nacelle maximum set of parameters has been used to describe the direction as well as active power and grid/brake status. character of the inflow to the turbine rotor (due to limitations in complexity of DAS not all of these At the two German test turbines the data acquisition parameters can be established for each of the test sites): system (DAS) has been designed in persuit of an improved load monitoring system featuring remote control and • Statistical descriptors for wind speed namely mean logging all data in one data logger network. The DAS value (U), standard deviation (cru, crv and c»), performs true on-line data reduction into statistics using skewness (s) and kurtosis (k) of the wind speed rainflow counting and time at level statistics avoiding time distribution series recording. • wind shear exponent (a) • wind velocity inclination (E) In the current measurement campaign the time base for the • Turbulence length scale (Lu) individual monitoring periods has been chosen to be 1 day • Coherence decay factor for the longitudinal wind speed as a compromise between including low cycle fatigue component between two points in the vertical relevant phenomena on one hand and on the other hand direction. shortening the measurement campaign through the 3.2 Analysis • matrix mean value equivalent to the center of gravity's position on the To establish the relative importance of these external diagonal of the of the load cycle matrix. conditions parameters statistical techniques have been used. • matrix rms value 3.2.1 Multivariate Regression Analysis parameter rating the dispersion of the load cycle distribution The equivalent load ranges for a body of N rainflow counts are expressed as a function of the independent inflow • matrix irregularity parameters given above: is defined by the number upwardly directed mean level crossings divided by the number of local maxima in a load sequence. This parameter rates the y(xj = f,akXk(x0 + E(Xi) , i-IN degree of symmetry of the load history with respect to k-l its mean level (e.g. a load sequence that vibrates (1) around the mean value will lead to an IR = 1); In where terms of RFC matrix shape IR says something about v(Xi) = equvalent load range how the load cycle counts are distributed in the = coefficient of k* independent variable matrix with respect to the matrix mean value. 1R=1: = value of k* independent variable at Xj large numbers of counts are found around the matrix E(xO = associated error mean for varying amplitudes. In order to calculate the coefficients of this function the • matrix change parameter MCP[8]: multivariate regression analysis as introduced in (7] and MCP is a parameter sensitive to changes in the shape successfully applied in [6] has been adopted. The of the matrix. It is expected to become a constant assessment of the regression findings is attained through when the matrix's shape stabilizes. the dependence coefficients Sk which represent the percent increase of the equivalent load range by an increase of one These parameters are calculated for each monitoring data of the inflow parameters by one standard deviation: set. Their relation to the inflow parameters is to be investigated during the measurement campaign. Special interest is also focussed on the MCP as it will serve as a £ (x*(x,)-X*)' measure for matrix validity. i'l Sk = 3.2.3 Normalised Load and Wind RFC Amplitude Spectra N-J (2) There is a desire to find a general method by which the where fatigue spectra, each obtained for different external Sk — dependence coefficient conditions, can be standardized. Reversing the X(x,) = mean value of k* independent variable standardization will then enable to estimate the fatigue 01 loading associated to any given set of parameters G» = std. dev. of k independant variable describing the changed external conditions. 3.2.1 RFC Amplitude Spectrum Modelling Hans Bergström introduced and applied this approach on wind RFC spectra for different sites [9]. He normalized the In an attempt to get insight into the statistical modelling of range of each wind variation cycle with the standard the fatigue process the RFC amplitude spectrum is deviation of the wind, cru , calculated for each one hour considered. As a first step an appropriate distribution type period. Encouraged by his promising results the same to model the amplitude spectrum is chosen. Four two- approach is adopted here to normalize measurements of parameters continuous distributions namely the Gamma, blade root bending moments from the Alsvik data base as a the Weibull ,the inverse Gaussian and the lognormal test case. In order to assure validity as a scaling pajameter distribution will be considered. For the estimation of the the time period of one hour as a basis for standard distributions' parameters a weighted maximum likelihood deviation determination is maintained and start/stop events scheme will be employed in order to take into account the are excluded. relative weight of different cycle amplitudes in the lifetime estimation. Furthermore, the parameters of the chosen Li case of the edgewise bending moment its variation is distribution are modelled using the parameters describing dominated by the sinusoidal effect of the gravitational the incoming flow and the system characteristics. This will bending moments, as the rotor rotates. In fact the be done via multiple regression method, not necessarily sensitivity of the standard deviation in the edge moment to linear, depending on the form of correlation between the the external conditions is greatly diminished. In order to parameters of the chosen distribution" and the external get around this problem the gravitational contribution to parameters. the edgewise bending moment has been excluded. After this is done, it's only the aerodynamical forces that has a 3.2.2 Rainflow Matrix Rating contribution to the edgewise bending moment, and If the rainflow matrix representation of a load quantity is consequently, the calculated standard deviation will be directly obtained by on-line data reduction appropriate affected by the stochastic nature of the wind instead of the parameters describing the quality of the matrix have to be deterministic gravitational force. picked. In the framework of this project the following descriptive parameters have been adopted: 4. FIRST MEASUREMENTS & EVALUATIONS The following are the main conclusions: • Fatigue is mainly governed by standard deviation of 4.1 Parameter Identification for Equivalent Load wind speed. Range Representation • The lateral and vertical turbulence ratios are imposing an additive effect on fatigue; these magnitudes are The parameter identification procedure was applied for all strongly connected to complex terrain characteristics. WT loading parameters. Figure 1 shows a sample • Mean wind speed appears as a strong parameter only sequence of 10-min time series records as used for this in the high wind speed ranges. arJysis. • Wind inclination, wind shear, wind speed distribution 15 magnitudes and length scale are imposing secondary

I" - effects. 12 f 3*£ i > TACKt 500LW Poromeier idenlKicollon V »t. 1 n * i o \é.* v» 0.15 - K/ E 0 c v **• ni ••1 "hi "^ T is Model poromeiers 0.10 - t •2 1 J 0.05 - PS *. Ä 1 1 Fig 2: Dependence coefficients for lHz Leq of flap, 0.00 - 1 bending moment 1 f 1 i 72 144 216 2B8 360 432 504 576 648 720 7S2 864 lOmin record E 200 Wind speed bin U=6-J0m/s U=10-14m/s lM4-18m/s »i Parameter mean SDV mean SDV mean SDV 150 | Mean wind speed (TJ) 8.36 1,08 11,75 1,21 15,68 1,12 Wind shear exp. (a) 0,061 0,094 100 i* i 0.072 0.038 0,085 0,036 #•. r / 1 ä Wind inclination (e) -3.96 2,06 •1.75 1,51 -0,812 0,999 1 i 50 h 1 SDV wind speed (au) 0,76 0,27 U2 0,31 1,51 0,244 5 t i Bl lateral turb. ratio (c/a,,) 0,99 0.22 0,835 0,147 0,807 0,118 —h -fj— Vertical lurt>. ratio (a^aa) 0,72 0,17 0,613 0,089 0,611 0,07 72 144 216 288 360 432 504 576 648 720 792 864 10min record Wind sp. skewness(U,) -0,112 0,319 -0,325 0,277 -0,457 0,251 E 300 Wind sp. kurlosis (Uk) 0.062 0,54 -0,114 0,48 -0,098 0,49 É 1 Turb. length scale (U.) 49.2 18,7 63,9 22,5 72,3 21,0 \ t i 200 ? Table 2. Independent parameter variation. * t* i. • \ * •i v r i 4.2 Matrix Rating

In the following an impression is given about the quality of data as it is acquired in the current measurement I _ mm campaign. A complete data set consists of RFC matrices 72 144 216 288 360 432 504 576 648 720 792 864 for all load quantities and the wind speed. Each day one 10min record such data set is read out from the DAS's memory. The idea Figure I. Measurement sample sequence of 10-min data is to continuously monitor the load quantities and to obtain sets snapshots of the actual status when reading out Through The results of the multivariate regression for the IHz subtraction of the data set of day N-1 from the data set of flapwise equivalent load (m=12, m being the slope of the day N RFC matrices for each individual day may be chosen material S-N curve) are presented in figure 2. The obtained. Subsequently the matrix parameter as introduced chosen descriptive parameters are listed in table 2, where above are calculated. Figures 3-5 depict the cumulative their basic statistics are included. It must be noted that the development of the flapwise blade root bending RFC parameter increment, on which the dependence matrix together with the matrix parameters. coefficients are estimated, equals to the standard deviation found in that table. matrix rms for the wind speed, each quantity normalized M=112 kNm, IR = 0.349, RMS = 35,79 kNm, MCP=9474228 by its mean value. The observation time period is again 21 days and identical with that of the load RFC matrices in figures 3-5. As could be expected from the parameter identification exercise they behave in a similar way.

50000

-300

To • (lapwise

From - flapwise (kNm,

300000 400000 600000 100000 1000000 1K0OOO 1400000 1000000 1tt0O00 tbpttd tin» (Met) 900 900 Figure 6: Relative RFC matrix parameter Figure 3: RFC matrix flap, bending moment, 3 days However, the matrix change parameter MCP is not yet

M=121 kNm, IR = 0.297, RMS = 43.38 kNm. MCP=9799685 seen to stabilize. Once its value stay constant the matrix is believed to be stationary and thus representative.

50000 .300

To • flapwis

0 300000 40000D 6W0OO I00CO0 IO0O000 1300000 1400000 IM0000 1100000 From - flapwise (kNi

Figure 7: Matrix Change Parameter 900 900 4.3 Normalized Load and Wind RFC Amplitude Figure 4: RFC matrix flap, bending moment, 11 days Spectra

M=102 kNm, IR = 0.235. RMS = 44.18 kNm, MCP = 503S108 Situations with and without wake operation have been studied. The qualitative effect of normalizing the load spectra is the same in both cases. This indicates that the load ranges scale on the load standard deviation in the same way, independently of the turbine operating in free or wake inflow. Figure 8 shows the flap load spectra of a period which includes wake effects making up for a complete measuring period of one day. Figure 9 depicts the 50000 spectra after normalisation. The standard deviation used •300 for normalisation has been calculated for 1 hourly periods.

flapwise

From - flapwise {kNm]

900 900

Figure 5: RFC matrix flap, bending moment, 21 days It is expected that the RFC matrices stabilizes as time proceeds. Figure 6 shows developments of equivalent load and matrix rms for the flapwise blade bendig moment and Fig 8: Flapwise load spectra. Each curve represents the accumulated RFC of one day. DAS = Data Acquisition System 6. REFERENCES [1] Monitoring Fatigue Loads on Wind Turbines Using Cycle Counting Data Acquisition Systems : final report for the Commission of the European Union, Directorate General XII for Science, Research and Development / Söker, Holger [ed.] ; Deutsches Windenergie-Institut ; Flygtekniska Foersoeksanstalten ; Centre for Renewable Energy Sources . - Wilhelmshaven: DEWI, 1995.-(JOU-CT92-0175) [2] Matsuiski ,M.;Endo,T.: Fatigue of Metals Subjected to Fig 9: Normalised flapwise load spectra. Varying Stress: paper presented at the Kyushu district meeting of the Japan Society of Mechanical Engineers, After the gravitational forces have been excluded from the March 1968. edgewise moment, normalization of the edge moment [3] Poppen, Maria; Dahlberg, Jan-Ake: Fatigue Loads on works as good as normalization of the flap moment. Wind Turbine Blades in a Wind Farm / Flygtekniska There is, however, a fairly large scatter in the normalised Foersoeksanstalten . -Bromma: FFA, 1992. load spectra, indicating that there exist additional -(FFA-TN-1992-21) parameters having an effect on the spectra, not treated [4]Hinsch, Ch.; Westermann, D.: Leistungskurven- here. It is also possible that the amount of statistical data is vermessung mit Hilfe des Gondelanemometers. DEWI- to small, i.e. coincidence of the normalized spactra could Magazin(1996)8,p58-64 be better if each curve had included sampled data for a [5] European Wind Turbine Standards : final report; vol.2 longer period than one day. (Load spectra for wind turbine design) / Brussel: EG, 1996. - getr. Zähl. - (JOU2-CT93-0387) The discrepancy is largest for the least frequent/high [6] Mouzakis F., Morfiadakis E., Dellaportas P.: amplitude cycles, i.e. it is obvious that the standard Parameter Identification on Power Performance of deviation does not work well for the low cycle fatigue area. Wind Turbines Operating at Complex Terrain : 2ni 4.4 Low Cycle Fatigue Effects EACWE, Genova, 1997. [7] Mouzakis F., Morfiadakis E., Fragoulis A.: Complex Terrain Wind-WT Parameter Identification and An estimation of low cycle fatigue effects has been carried Quantification : Wind and ' WT measurements, out by CRES and has been published in [10]. MOUNTURB final report (JOU2-CT93-0378), vol. II, 1996. 5. CONCLUSIONS [8] Reinke, Wilhelm: Ein Beitrag zur Extrapolation von Wechselbelastungen aus zweiparametrischen A statistical approach has been utilised for the parameter Zähherfahren, - Dilsseldorf: VDI-Verl., 1988. - (VDI identification task. The parameters investigated describe Fortschrittsberichte; 5,151) the deterministic part of the wind, the main turbulent [9] Bergström,H; Ganander, H.; Johannson,H.: Wind characteristics, the turbulence length scale as well as the Description for Designing Wees, Based on RFC wind speed distribution. The effect of the above Evaluated Wind Measurements . In: Wind energy : parameters was captured and quantified. technology and implementation ; proceedings of the Parameters describing the quality of RFC matrices have European Wind. Energy Conference, EWEC '91, been adopted. The monitoring procedure has been started Amsterdam, The Netherlands, October 14-18, 1991. and the first data sets have been evaluated. The Amsterdam [u.a.]: Elsevier, 1991. - S. 762-766 appropriateness of the chosen matrix parameter is [10]Mouzakis F., Morfiadakis E.: Identification of Low indicated but as to the early stage of the measurements no Cycle Effects on Wind Turbine Component Lifetime conclusive statements can be made at this point. Estimation. In: Proceedings of the BWEA'97 held at Edinburgh, July 16-18,1997 [forthcoming] Normalization of the RFC load spectra by the load standard deviation works well for the high cycle fatigue Acknowledgement region. However, for cycles with low frequency and large amplitude there is a large scatter in the normalized spectra. Since the fatigue of the turbine is sensitive to these cycles, The work presented in this paper has been partially the method has to be further developed in order to be able supported by the European Commission, DGXII Non to scale measured RFC spectra for changed standard Nuclear Energy Programme under contract nr. JOR3~ deviations of the observed load quantity. Moreover, the CT96-0I03. standard deviation of the load quantity shall be mapped to The authors would like to express their thanks to Mr. the standard deviation of the wind speed in order to enable Bädeker, private operator in Oldorf, Germany, Mr. Klug, scaling of the load RFC with the wind standard deviation Mr. Bruns and Mr. Frerichs from SW Emden as well as as a scaling parameter. PPC in Greece. Also the authors wish to thank TACKE Windtechnik and 7. NOMENCLATURE especially Mr. Siebers for their valuable contribution to RFC = Rain Flow Counting the project work. FFATN 1997-48

Appendix 7

Tests and Design Evalution of a 20 kW Direct-Driven Permanent Magnet Gener- ator With a Frequency Converter

Anders Grauers, Ola Carlson, Erik Högberg, Per Lundmark, Magnus Johnsson, Sven Svenning TESTS AND DESIGN EVALUATION OF A 20 kW DIRECT- DRIVEN PERMANENT MAGNET GENERATOR WITH A FREQUENCY CONVERTER

A. Grauers*, O. Carlson*, E. Högberg*, P. Lundmark*, M. Johnsson** and S. Svenning** *Depaitment of Electric Power Engineering, Chalmers University of Technology, S-412 96 Göteborg, Sweden E-mail: [email protected]. [email protected] Phone: +46 31-772 16 37, Fax: +46 31-772 16 33 **Pitch WindAB, Fritslav. 34, 511 57 Kinna, Phone: +46 30216165 ABSTRACT This paper presents the tests and design evaluation of a 20 kW, direct-driven, permanent magnet generator with a frequency converter. The electrical system is designed for a gear-less wind turbine with a diameter of 14 m and a passive blade pitch system. The generator has an air gap diameter of 0.9 m, a stator length of 20 cm and a total weight of 610 kg. The main shaft and bearings are integrated into the generator design. The results of the measurements and the theoretical evaluation show good system performance and high efficiency. It was also found that this direct-driven generator is more efficient than a conventional four-pole generator equipped with a gear.

1 INTRODUCTION theoretical predictions. The evaluation focuses on generator voltage, resistance, inductance, losses and Despite the steady increase in the power rating of wind peak power. turbines up to the multi megawatt range, there is still an interesting market for small wind turbines up to about 20-30 kW. In developing countries these small wind 2 THE WIND TURBINE turbines are applied in autonomous energy systems, while in the developed world an increasing interest can The wind turbine of Pitch Wind is equipped with a be noted for farms or individual homes. unique passive blade pitch control system. Figure 1 shows the wind turbine and Table 1 presents its data. Aerodynamic forces will automatically pitch the blades The development of new permanent-magnet material when the turbine reaches its nominal speed. With this and the reduction of the cost of power electronic system, together with a variable speed electrical components offer the opportunity to design cost- system, it is possible to control the upper limit of the effective wind turbines with a direct-driven generator. power in a very accurate way and no overshoot of the However, the direct-driven synchronous generator needs power will ever occur. This way of controlling the damping, either with mechanical springs and dampers power is very seldom used. The passive blade pitch or, preferably, with an electrical damping, i.e., variable system also works as an emergency system which speed operation. By using direct-driven generators, limits the maximum speed of the turbine in the event of which are optimized for low speed operation, the loss of grid. The turbine blades are individually performance (efficiency, reliability) of the systems can suspended in their own flap-regulation shaft joint, so be improved whilst the costs can be reduced. The that the turbine blades can freely move back and forth assembly of the drive train is also simplified compared to permit flap action while in operation. Pitch Wind with a conventional drive train with gear and generator. uses a tubular reinforced concrete tower. The tower i s

The tests and design evaluation of a 20 kW direct- driven generator with a frequency converter are reported in this paper. The direct-driven generator is connected to the grid via a frequency converter system. This electrical system allows a variable speed of the generator and turbine and it a provides damping of the generator.

The generator, as well as the wind turbine, are designed and built by Pitch Wind AB. Laboratory tests of the drive train have been carried out at Chalmers University of Technology. The tests were performed within the whole speed and power range. On the basis of several years of design experience, the design of the generator is evaluated and performance is compared with Figure I. The Pitch Wind turbine. Table 1: Main data of the wind turbine. Rated power 20 kW Turbine diameter 14 m Number of blades 2 Hub height 30 m Tower Concrete originally developed for high voltage line towers and it is made in large series. Because of the passive pitch control system the turbine needs no control computer, but there is a control computer integrated into the frequency converter.

The connection of the wind turbine to the grid can be made within the owner's local grid, i.e., on the owner's Figure 2. The nacelle and the direct-driven 20 kW side of the electrical energy measurement equipment. At generator. this point of connection, the grid is very weak and the type of inverter is high, more than 98 %, but the risk for flicker disturbance is obvious. Since the wind thyristor inverter will consume reactive power and turbine is equipped with a frequency converter for produce current harmonics. variable speed operation, there will be no power variations at rated power and only slow power The power from the wind turbine is controlled by the variations below rated power. Consequently, this control computer which is integrated into the thyristor generator system will cause much less voltage flicker inverter. The fifth harmonic in the grid current is than a conventional generator system. filtered and the filter also produces reactive power to improve the power factor of the grid currents. 3 THE ELECTRIC SYSTEM 4 PERFORMANCES 3 .1 The Generator The generator is a permanent magnetized synchronous The laboratory system and tests of the generator are generator connected to the grid via a frequency converter. reported in more detail in [1]. The generator is manufactured with NdFeB-magnets on the rotor. It has 66 poles and generates a three-phase 4 .1 Laboratory Set-up armature voltage. The nominal speed is 75 rpm. The size Figure 3 shows the laboratory system. The generator i s and weight of the generator are as follows: driven by a 40 kW dc machine. The nominal speed of the dc machine is 1500 rpm and therefore a gearbox of Active length of stator and rotor: 0.20 m ratio 21:1 is mounted between the generator and the dc The internal diameter of the stator: 0.92 m machine. The generator shaft speed and torque are Air gap: 2.1 mm measured, and thereby the input power. In addition the Outer diameter: 1 m generator armature voltage, current and active power are Outer length: 0.4 m measured by a three-phase power meter which measures Generator weight: 610 kg the frequency of the voltage as well. The power to the grid is also measured with a second power meter. A photo of the nacelle and the direct-driven generator is shown in Figure 2. During the tests the whole generator was hung on the generator shaft. Only a lever and a steel wire prevented 3.2 The frequency Converter the generator housing from rotating, see Figure 4. In The alternating current from the generator is converted this way, the shaft torque could easily be measured by a into a dc current by a six-pulse diode rectifier. The diode strain gauge, which measured the force in the wire. rectifier is used because of its high efficiency, over 99 %, and its low cost. In order to smooth the 4.2 Results of Measurements dc current, an inductance (of 20 mH) is placed between The impedance of the generator is very important, the rectifier and the inverter. The dc-current is then since it causes a voltage drop which limits the power of converted to an alternating current of grid frequency by the generator. The armature resistance was measured to a three-phase thyristor inverter. The efficiency of this 0.54 n/phase. The inductance was estimated from the

DC machine PM machine Smoothing inductance

Diode Thyristor rectifier inverter Figure 3: The laboratory system. has been slightly changed. The air gap is reduced and the stator is longer, to improve the peak power of the Strain gauge generator. The generator will still be used for 20 kW rated power, however, copper losses will be reduced.

5 EFFICIENCY COMPARISON WITH A —Gear DC 3 machine CONVENTIONAL DRIVE TRAIN In this section, the energy efficiency of the direct-

i | P M generato r 1 driven generator system is compared with that of a conventional fixed-speed direct grid-connected 4-pole induction generator with gear. Rear wiew Side view The direct-driven generator has much higher copper Figure 4. The laboratory set-up for measuring losses than a conventional four-pole generator, while the generator shaft torque. the core losses are, instead, much lower. This difference makes the direct-driven generator less efficient than a dc voltage-current characteristic to 7.0 mH/phase. conventional generator at rated load, but at part load it Figure 5 shows the dc voltage and power as a function is, instead, more efficient. In Figure 7 the measured of the dc current at nominal speed. The power increases losses of the variable-speed direct-driven generator and almost linearly with increasing current as the dc current its frequency converter are plotted. In the same increases from 0 to about 20 A. At high currents the diagram, the losses of the conventional induction voltage drop becomes so high, due to the armature machine and its gear are also shown. The Figure shows impedance, that the power decreases. This drop limits that the direct-driven generator system has lower losses the power. At 75 rpm the power maximum is 19.6 kW than the conventional system below 13.5 kW input at a dc current of 74 A, for the generator prototype. power.

The maximum power of the direct-driven generator To calculate the energy efficiency, the approximate varies depending on which type of rectifier is used. If a power curve in Figure 8 is used. Since this wind energy transistor rectifier is used, the maximum power for this converter is designed for low wind speed sites, a site generator will be more than 50 % higher compared with with an average wind speed of 5 m/s has been the maximum power when a diode rectifier is used. The considered to be typical. difference in maximum power is discussed more in [2]. The main reason for the higher power is the transistor The investigated wind energy converter, on a site with rectifier's ability to provide the generator with reactive an average wind speed of 5 m/s, will stand still about power to compensate for the internal reactive voltage 25 % of the time, because the wind speed is too low. It drop in the generator. Of course, a transistor rectifier is will operate 60 % of the time below 13.5 kW and only more expensive than a diode rectifier. 15 % of the time above 13.5 kW. Because of this power distribution the direct-driven generator will lead to The efficiency of the generator and frequency converter lower energy losses than the conventional system, is only 86% at maximum power, but it is, which is since it has low losses at partial load. The annual more important, over 90% at part load. Moreover the energy efficiency, in Table 2, is calculated for both efficiency is high at low power since the generator then drive trains as described in [3]. Since the probability operates at low speed. Figure 6 shows the efficiency as for different power depends on the average wind speed a function of output power for three different speeds. of the wind turbine site the energy efficiency also changes with site. The tested generator was the first prototype. To improve the performance, the series produced generator

1 500 20 0.9 450 18 s' Power : 0.8 400 16

0.7 350 14 ^^^ Vollagei £300 ... . 12 1 s-0-6 !.... ^P>K.. 10 Jo.5

D c voltag e 0.4

8 Dcpowe r (

560 6 0.3

0.2 100 • : 4

2 SS : 0.1

9 10 20 30 40 50 60 70 80° C 10 Dc current (A) Power input (kW) Figure 5. Dc voltage and power as a function of Figure 6. Efficiency of the generator and frequency dc current at 75 rpm. converter. 3 -T-- Table 2. Approximate annual energy efficiency of the two drive trains at different sites. Avera ge wind sp sed 4 m/s 5 m/s 6 m/s Direct-driven gen. 85.0 % 85.3 % 85 2 % & frequency conv. O -!- Induction O 5 10 15 20 71.5 % 80.0 % 83 3 % Input power (kW) generator & gear -Direct-driven generator and frequency converter Increased energy -Constant speed induction generator and gear production for +19% +6.6 % +2.3 % direct-driven gen. Figure 7. Losses of the generator systems as functions of input power from the wind turbine. inductance and, therefore, the generator can be made with deeper slots for the windings. Deeper slots decrease copper losses and allow higher force densities in the air gap, leading to a smaller generator.

The thyristor inverter can be replaced by a transistor inverter which will reduce the grid current harmonics and improve the quality of the delivered power.

10 12 14 7 CONCLUSION Wind speed (m/s) The results of the measurements and the theoretical Figure 8. Approximate power curve for the 14 m wind evaluation show that the system with a permanent- turbine. magnet low-speed generator and a frequency converter has good system performance and high efficiency. By using a direct-driven generator, instead of a conventional induction generator and gear, the drive It has also been found that this direct-driven generator train losses are reduced. The results in Table 2 show is more efficient than a conventional four-pole that this wind energy converter will produce generator equipped with a gear. approximately between 2 and 19 % more electricity since it is equipped with a direct-driven generator instead of a conventional drive train. The increased 8 ACKNOWLEDGEMENT turbine efficiency caused by the use of variable turbine speed has njot been included in these figures, since the The authors would like to thank Mikael Alatalo for his same approximate power curve has been used for both valuable electrical design work with the generator. The drive trains. The different power curves for variable- financial support given by the Swedish National Board and constant-speed turbines will make the difference for Industrial and Technical Development and Elforsk is between the systems even larger. gratefully acknowledged. The authors would also like to thank Bo Lagerkvist for his valuable laboratory work. The energy efficiency of the induction generator system shown in Table 2 can be increased, since special designed low-loss induction generators can 9 REFERENCES have lower losses than the standard induction machine assumed here. However, also the efficiency of the [1] Högberg E. and Lundmark P. "Design and Test of an direct-driven generator system can be improved. If the Electrical System for a Direct-Driven Permanent- direct-driven generator is designed for a forced- Magnet Generator for a Wind Turbine" Göteborg, commutated rectifier it will on most wind turbine sites, Sweden, Chalmers University of Technology, School almost certainly, have an energy efficiency which is of Electrical and Computer Engineering, M.Sc. Thesis higher also than that of a low-loss induction generator. No. 16E, October 1997.

[2] Grauers, A. "Design of Direct-driven Permanent- 6 FURTHER DEVELOPMENT OF THE magnet Generators for Wind Turbines". Göteborg, DRIVE TRAIN Sweden, Chalmers University of Technology, School of Electrical and Computer Engineering, Technical, Report No. 292, November 1996. The direct-driven generator used in the presented wind energy converter is very good. Still there is potential [3] Grauers A. "Efficiency of three wind energy for further improvement. The size of the generator can generator systems", IEEE Transactions on Energy be reduced significantly and the efficiency can be Conversion, Vol. 11, No. 3, pp. 650-657, September increased by designing the generator for a forced- 1996. commutated rectifier instead of a diode rectifier. A forced-commutated rectifier allows higher generator FFATN 1997-48

Appendix 8

The Rating of the Voltage Source Inverter in a Hybrid Wind Park With High Power Quality

Jan Svensson THE RATING OF THE VOLTAGE SOURCE INVERTER IN A HYBRID WIND PARK WITH HIGH POWER QUALITY Jan Svensson Department ofElectric Power Engineering, Chalmers University of Technology, S-412 96 Göteborg, Sweden

ABSTRACT A hybrid wind park consists of wind turbines which have different electrical systems: directly connected induction generators (1G) and variable-speed electrical systems. One of the variable-speed electrical systems has a voltage source inverter (VSI) connected to the grid, the others have grid-commutated inverters (GCI). The VSI can be used for reactive power compensation and active filtering, in addition to converting wind power. These additional features cause an increase of the VSI rating. The degree of over-rating depends on the wind park configuration. In a three-turbine wind park, consisting of an IG, a GCI and a VSI of equal rated active power, the rated current of the VSI becomes 1.33 pu for the rms-current comparison and 1.73 pu for the peak-to-peak current comparison. When a 5th-harmonic passive shunt filter is used to cancel the 5th harmonic current from the GCI and to produce the mean consumed reactive power of the wind park, the rated power of the VSI becomes 1.06 pu for the rms-current comparison and 1.2 pu for the peak-to-peak current comparison.

1. INTRODUCTION The interest in the variable speed operation of wind turbines has increased in recent years because of the potential to improve the total wind turbine system performance. Usually, passive shunt filter induction -d grid-commutated inverters (GCI), equipped with thyristors, are generator commutated used on the grid side of the electrical system. The cost for these rectifier inverter converters is rather low and the efficiency high. However, there are two problems associated with GCIs: low-frequency —AP current harmonics are injected into the grid and the GCI consumes reactive power. Wind parks are often located in rural point of I synchronous common areas where the grid is often weak, i.e., the grid has a relatively voltage generator connection source low short-circuit power. A wind park connected to a weak grid rectifier will increase the voltage variations, and variable speed systems inverter with GCI will also increase the voltage harmonic distortion. Today, these problems are solved by increasing the short- circuit power of the grid. Another solution is to equip all wind turbines with forced-commutated voltage source inverters /synchronous weak grid generator (VSI), but the VSI costs more and has a lower efficiency than Figure 1: The hybrid wind park configuration. the GCI. For the variable-speed system with the GCI, two DC-link The benefits of all systems can be utilized by a hybrid wind voltage control strategies can be used to control the SG. They park consisting of wind turbines which have different electrical are the constant flux mode (CF) and the optimal efficiency (OE) systems: directly connected induction generators (IG) and mode [1,2]. variable-speed electrical systems. One of the variable-speed electrical systems has a VSI connected to the grid, the others 3. THE POWER QUALITY OF THE TURBINES have GCIs. A. Reactive Power This paper deals with a hybrid configuration of a wind park, Since the reactive power consumed by the IG and the GCI consisting of up to 3 turbines. The investigation is a cannot be controlled, the voltage level of the PCC will vary if continuation of [1], in which the focus was on reactive power the grid is weak. The two different control methods of the GCI compensation and the voltage level at the point of common will also affect the grid voltage differently. Figure 2 shows the connection (PCC). Here, the emphasis is on the rated current reactive power of the IG and the GCI as a function of the active of the VSI when it converts active power and simultaneously power. The units are in the process of consuming reactive operates as an active filter and a reactive power compensator. power and delivering active power to the grid. For a typical The rated active powers of the VSI, the IG and the GCI are 400 kW IG, the reactive power varies from approximately assumed to be equal, and the total reactive power to the grid is -0.26 pu to -0.46 pu. The reactive power of the GCI goes set to zero to obtain a unity power factor. The required size of from 0 to -0.36 pu. The CF-mode has the largest reactive the VSI in different systems and operating modes is power span, from 0 to -0.46 pu. The reason for the different investigated. reactive power functions for the two types of GCI control methods is that the DC-link voltage is higher in the OE mode at 2. THE SYSTEM CONFIGURATION intermediate wind speeds. The minimum advance angle of 20° The hybrid wind park configuration is shown in Figure 1. The is a trade-off between the DC-link voltage and safety margins variable-speed systems use synchronous generators (SG) of commutation failures. together with rectifiers. A 5th-harmonic passive shunt filter (PSF) is connected to the PCC. o- i for different wind park configurations together with the wind GCI(C >E) speed corresponding to the mean reactive power. -0.1 Table I: The maximum, the minimum and the mean reactive A power (generator reference) for different configurations. The Jr -0.2- y L wind speed varies between 0 and I pu. V;~ , Configuration 2™ [pu] wind speed [pu] | -0-3- V. ^^•^ öm* IP"] Qmm IP"! r corresponding •° Ö,,,,,,, ——• GCI(CF) IG -0.46 -0.26 -0.36 0.89 * -0.5- GCI(CF) -0.46 0 -0.23 0.50 GCI(OE) -0.36 0 -0.18 0.44 -0.6- IG + GCI(CF) -0.84 -0.26 -0.55 0.56 0.2 0.4 0.6 0.8 IG+GCI(OE) -0.82 -0.26 -0.54 0.84 Active power [pu] An example of the time variation of the currents is shown in Figure 2: The reactive power (generator reference) as a function Figures 3 and 4, when the wind park consists of a VSI, an 1G of the active power for the IG and the GCl. The GCl uses CF and and a GCl. The VSI operates as a reactive power compensator OE modes. and an active filter in addition to active power generation. In The PSF produces a constant reactive power and is designed to Figure 3, the GCl current and the grid voltage for wind speeds deliver the mean reactive power consumed by the GCl and the of 0.7 pu and 1.0 are presented together with the grid voltage. IG, in the wind speed span from 0 to 1 pu. Due to the vector The resulting VSI current and its fundamental component are current controller of the VSI, the reactive power can be shown in Figure 4. The peak-to-peak value of the VSI current controlled independently of the active power. at a wind speed of 1.0 pu becomes 1.73 pu, and the fundamental current component becomes 1.3 pu. B. Low-frequency Harmonics Consequently, the active filter option requires a considerable The GCl produces low-frequency current harmonics with increase of the rated current of the VSI. amplitudes dependent on the active power level. In the following analysis, the phase currents of the GCl will have a 1.5 quasi square wave form due to a smoothing dc-link inductor [3]. Harmonics up to the 31st order are taken into consideration. The vector-controlled VSI can act as an active shunt filter and cancel the current harmonics of the 5th, 7th 11th and 13th order. Furthermore, the VSI and its control system are assumed to operate ideally. Of course, active filters can cancel higher-order harmonics, but then the losses of the VSI > .1-4 4. METHOD OF ANALYSIS GCI(PF), 0.7pu -1.5 _l j The required rated power of the VSI is analysed in various 0.01 0.02 0.03 0.04 cases: when the VSI acts as an energy converter, a reactive Time [s] power compensator, an active filter or as a combination of Figure 3: The GCI(CF) currents and the grid voltage for wind these acting modes. speeds 0.7pu and 1.0pu. The current rating of the VSI can be determined either by the rms-value or the peak-to-peak value of the current. For 2- sinusoidal currents, the two methods result in the same VSI (1),1.0 >u^^___- VSI,1.0pu 1.5- A converter rating. If the current from the VSI is distorted, the ^ two methods will differ. Depending on the type of valves the A VSI is equipped with, one of the two methods is more "åX\ / /.. appropriate to use. The rms current of the VSI is a degree of 0.5- A total losses in the valves. Thus, the rms-current value of the iw.. v.. VSI for non-sinusoidal currents can be compared with the rated vL sinusoidal current of a standard VSI. The peak-to-peak current -0.5- ^...rtrff \ value of the VSI should be used when the maximum current of -1 " the valves is the limiting factor. In this case, the peak-to-peak / i\, 0.7pu current of the VSI must be compared with the sinusoidal peak- -1.5- V vsi( l),0.7pu ' to-peak current of a standard VSI. The most usual valves in -2- medium-sized inverters are IGBTs. The IGBT does not have a 0.01 0.02 0.03 0.04 maximum current limit, as the IGCT has [4], but the IGBT Time [s] must be protected from overheating. Table 1 displays the Figure 4: The VSI current and its fundamental component, maximum, minimum and the mean consumed reactive powers VSI(l), for wind speeds 0.7pu and 1.0pit. 5. INVESTIGATED WIND PARK CONFIGURA- current of the VSI is 0.87 pu without the PSF, and by using TIONS the PSF it decreases to 0.34 pu, a reduction of 6] %. Ten different wind park configurations were investigated as 0.6 described in Table 2. Configurations A and F correspond to the active power production combined with the reactive effect compensation or active filtering. In configurations B to E, the VSI acts as an active filter and as a reactive power compensator. For the other variants (G to K), the VSI acts as an active power converter and the active filter and reactive power compensation modes alter.

Table 2: The different wind park configurations that have been simulated. The VSI compensates for the active power and/or reactive power and/or current harmonics. Variant VSI as Reactive power Orders of filtered (and total active power compensated by harmonics O 0.2 0.4 0.6 0.8 active power) converter the VSI Wind speed [pu] A (3 pu) yes IG, GCI Figure 5: The rms-current of the VSI as a function of the wind B (1 pu) no GCI 5,7,11,13 speed. Wind park variants B and C are displayed. C (1 pu) no PSF, GCI 7,11,13 D (2 pu) no IG, GCI 5,7,11,13 E (2 pu) no PSF, IG, GCI 7,11,13 F(1 pu) yes 5,7, 11,13 G (2 pu) yes GCI 5,7,11,13 H (2 pu) yes PSF, GCI 7,11,13 J (3 pu) yes IG, GCI 5,7,11,13 K (3 pu) yes PSF, IG, GCI 7,11, 13

6. RESULT EVALUATION Table 3 presents the maximum rated current of the VSI for wind speeds from 0 to 1 pu for the wind park configurations described in Table 2.

Table 3: The maximum current of the VSI for the wind speed 0.2 0.4 0.6 0.8 span 0 to 1 pu and for different system variants. Wind speed [pu] Variants Maximum peak value [pu] Maximum RMS value [pu] CF-mode OE-mode CF-mode OE-mode Figure 6: The rms current of the VSI as a function of the wind A 1.30 1.30 1.30 1.30 speed. Wind park variants D and E are displayed. B 0.74 0.73 0.52 0.47 C 0.47 0.47 0.28 0.27 The complete wind park (variant K), consists of an IG, a D 1.13 1.12 0.88 0.87 GCI(OE), a PSF and a VSI. The VSI operates as an active E 0.56 0.56 0.34 0.35 power converter, an active filter and a reactive power F 1.10 1.10 1.04 1.04 compensator. The rated current of the VSI becomes 1.06 pu G 1.38 1.38 1.10 1.10 with and 1.33 pu without the PSF, according to Figure 7. H 1.17 1.18 1.03 1.04 When introducing the PSF, the VSI must consume reactive J 1.73 1.73 1.33 1.33 power at low wind speeds, because the IG and the GCI 1.20 1.20 1.06 1.06 K consume less reactive power than the reactive power produced by the PSF. The VSI current is thus higher at low wind speeds A. The RMS-Current Comparison when the PSF is used. The crossing point for the VSI currents In this section, the rms-current comparisons of different wind is at a low wind speed, approximately 0.1 pu in Figure 7. At park configurations are in focus. If the wind park consists only low wind speeds the captured wind power is low, resulting in a of a GCI and a VSI which acts as an active filter and reactive minor current, and the losses become low or the turbines are power compensator (B), the rated VSI size becomes 0.52 pu shut off. for the CF mode and 0.47 pu for the OE mode. The rms- current of the VSI as a function of the wind speed is shown in If a PSF is used, the IG does not influence the rated current of Figure 5. By introducing a PSF (C), the VSI size will be the VSI, as demonstrated in Figures 7 and 8. The addition of reduced to 0.28 pu and 0.27 pu for the CF-mode and the OE- an IG increases the rated current of the VSI by 21 % without the mode, respectively. Furthermore, the OE mode of the GCI PSF but only by 3% with the PSF. Because of the almost decreases the current of the VSI below the rated wind speed, as constant reactive power demand of the IG, the current increase shown in Figure 5. The current reduction is important because is low. the mean wind speed is lower than the rated wind speed of the wind power plants, and lower rms-currents result in a greater efficiency. If an IG is added to the wind park (D, E), the consumed reactive power is increased. It is obvious, according to Figure 6, that the use the PSF is an advantage. The rated 1.4 I

1.2- J(OE) •^^^^ x// 1 " 0.8- z ! K(CF) v/ > 7/ 0.6- ;K(ÖÉ)/ V I U 0.4- V L* u > 0.2- 0 0 0.2 0.4 0.6 0.8 1 0.4 0.6 Wind speed [pu] Wind speed [pu] Figure 7: The rms current of the VSI as a function of the wind Figure 9: The peak-to-peak current of the VSI as a function of speed. Wind park variants J and K are displayed. the wind speed. Wind park variants J and K are displayed.

6. CONCLUSION 1.2 In a hybrid wind park, the VSI can be used for reactive power compensation and active filtering in addition to converting wind power. These additional features cause an increase of the VSI rating. The degree of over-rating depends on the wind park configuration. In a three-turbine wind park, consisting of an 1G, a GCI and a VSI of equal rated active power, the rated current of the VSI becomes 1.33 pu for the rms-current comparison and 1.73 pu for the peak-to-peak current comparison. When a 5th-harmonic passive shunt filter is used to cancel the 5th harmonic current from the GCI and to produce the mean consumed reactive 0.2 0.4 0.6 power of the wind park, the rated power of the VSI becomes Wind speed [pu] 1.06 pu for the rms-current comparison and 1.2 pu for the peak-to-peak current comparison. Figure 8: The rms-current of the VSI as a function of the wind speed. Wind park variants G and H are displayed. 7. REFERENCES B. The Peak-to-peak Current Comparison [1] Svensson, J., "Possibilities by Using a Sclf- The results of the peak-to-peak current demand of the VSI in commutated Voltage Source Inverter Connected to a the different wind park configurations have almost the same Weak Grid in Wind Parks," 1996 European Union characteristics as the rms-current demand. The peak-to-peak Wind Energy Conference and Exhibition, Göteborg, current is scaled up compared with the rms-current in the same Sweden, 20-24 May 1996, pp. 492-495. wind park configuration. The result of the wind park variant K is shown in Figure 9. The VSI current increases by 13% when [2] Grauers, A., "Synchronous Generator and Frequency the peak-to-peak current method is used. The largest increase is Converter in Wind Turbine Applications: System 74% for the wind park variant C. As in the rms case, the OE Design and Efficiency," Chalmers University of mode of the GCI reduces the current above 0.5 pu wind Technology, School of Electrical and Computer speeds, and the use of a PSF results in a lower VSI. Engineering, Göteborg, Sweden Technical Report No. 175L, May 1994. [3] Thorborg, K., Power Electronics - in Theory and Practice. Lund, Sweden: Studentlitteratur, 1993. [4] Gruening, H. E., Ödegård, B., "High Performance Low Cost MVA Inverters Realised with Integrated Gate Commutated Thyristors (IGCT)," 7th European Conference on Power Electronics and Applications (EPE'97), Trondheim, Norway, 8-10 September 1997. Proceedings.vol. 2, pp. 60-65. FFATN 1997-48

Appendix 9

Optimal Size of Wind Turbine Transformer

Åke Larsson Optimal Size of Wind Turbine Transformer

Åke Larsson M.Sc. Department of Electric Power Engineering, Chalmers University of Technology, S-412 96 Göteborg, SWEDEN e-mail: [email protected], telephone: +46 31 772 1642, telefax: +46 31 772 1633

ABSTRACT During the past decade considerable efforts have been put into the efficiency of wind turbines. Almost every component in wind turbines has been the object of optimisation. Still, when it comes to grid connection the transformer is very often oversized. In this paper the cost price of transformers is compared with the cost of transformer losses. The transformer losses are calculated from the wind probability curve and the power curve of the wind turbine. The temperature and thereby the lifetime of the transformer are calculated based on the IEC 354. The results show that an optimal transformer kVA rating is 20% lower than the rated power of the wind turbine.

1 INTRODUCTION 2 TRANSFORMER LOADS AND WIND TURBINE CHARACTERISTICS During the past decade the efficiency of wind turbines has increased. Almost every component in wind turbines has When wind turbines are connected to the grid ordinary been subjected to a technical and economic optimisation. standard transformers are normally used. Transformers up to 2 500 kVA are called distribution transformers indicating When it comes to connecting wind turbines to the grid, their field of application. In a distribution network, the load utility company or national regulations determine the normally consists of a base load and load peaks. The peak transformer sizes. For example, in Sweden the recom- load appears during hours of high load demand on the grid. mended transformer size for a 500 kW wind turbine is 800 If a transformer feeds offices, the peak load will appear kVA [1]. It seems that other countries have similar recom- during working-hours. On the contrary, if a transformer mendation. In papers from the UK, the Netherlands and the feeds residential quarters peak loads will appear during the USA, where the generator and transformer data are given, morning and during the evening. the same pattern of over-sized transformer can be found [2][3][4]. It seem as if the rated power of the transformer is somewhere between 1.5 to 1.7 times the rated power of the 2.1 Transformer Loads generator. When it comes to wind turbines the situation is quite different. Wind turbines do not have a load profile like ordinary Utility companies have a long experience of dimensioning distribution transformers. At a typical wind turbine site in transformers for transmission and distribution purposes. Sweden, the wind speed is usually about half of the rated Since wind turbines have a power production unlike both wind speed, see Fig. 1. transmission and distribution transformer loads, new dimension criteria must be derived. Hence, wind turbine transformers must, like all other components used in wind turbines, be chosen based on an optimal technical and economic basis. The technical optimum ought to be a transformer which can withstand the wind turbine load without forced ageing or at least having a lifetime greater or equal to the expected lifetime of the wind turbine. An economic optimum ought to be the lowest total price of the transformer, taking the cost of losses into account.

In this paper the cost price of transformers is compared to the cost of transformer losses. The losses for various trans- 10 12 14 16 18 20 22 24 formers at a wind turbine site are calculated. The power wind speed (m/s) production at the site is derived from the probability density Fig. 1: The weighting function of wind speeds at the of different wind speeds and the power curve of a wind harbour in Falkenberg, Sweden. turbine. In order to find the economic optimum of the transformer size, the cost of the losses are compared with In the figure, the weighting function of the wind speed at the cost price of the transformer. Finally, to fulfil the tech- the harbour in Falkenberg, Sweden, is shown. Worth men- nical optimum, the expected temperature and thereby the tioning is that the wind speed at this location is typical for lifetime of the transformer is discussed in accordance with the west coast in Sweden. Normally, wind turbines produce the International Standard IEC 354. rated power at wind speeds exceeding 12 m/s. As can be seen in the Figure, only during a small fraction of time, 3.2 Transformer Losses less than 10% of the year, is the wind speed equal to or Transformer losses consist of two parts, iron losses and greater than 12 m/s. Hence, transformers used in wind copper losses. The iron losses are also refereed to as no- turbines will only have a load equal to the rated power load losses and are due to the hysteres loop in the iron core. during less than 10% of the time annually. Most of the time The iron losses are proportional to the volume of the iron the transformer will operate at only part load. and constant for a given transformer provided that the voltage and frequency are constant. Since both the voltage and the frequency are fairly constant on a utility grid, the 2.2 Wind Turbine Characteristics iron losses are also constant. Hence, a small transformer has Different types of turbines used in wind turbines have smaller iron losses than a large one because of the amount different power characteristics. The average value of the of iron in the core. The iron losses are relatively small, power output from a pitch regulated wind turbine is smooth considering that iron losses occur 8 760 hours/year the total and does not exceed the rated power of the generator. iron losses calculated over one year are considerable.

Stall regulated wind turbines may, due to variations in the The copper losses are due to the current flow in the wind- density of the air and imperfections in the aerodynamics, ings and are obtained by: have a power output which sometimes is above the rated one. In the case of variations in the density, the cooling of r / v the transformer will not be a problem. Over-production due (1) to a high density of the air at a stall regulated wind turbine rated occurs at low temperatures. On these occasions, the trans- where Pcu.rated ar>d / rated are tne rated values given on former will also be better cooled by the ambient air. the name plate of the transformer. Pcu are the copper losses for a given current /, and R is the resistans in the trans- As pertains to imperfections in the aerodynamics, the gen- former windings. For a given apparent power, the current erator is protected from being overheated. The temperature will be constant provided that the voltage is constant. As of the generator in wind turbines is normally measured. can be seen in Equation 1, if the current is constant a small Hence, in the case of over-temperature in the generator, the transformer will have larger copper losses than a large one. wind turbine is stopped. The generator is also normally Since the wind speed according to Fig. 2 is usually about more sensitive to over-temperatures than a transformer. half of the rated speed, the transformer benefits more from This is due to a couple of hundred kilos of oil inside the low losses at low power than it does from low losses at transformer. rated power.

Hence, the choice of transformer must be based on the 3 COST OF TRANSFORMER AND LOSSES balance between iron losses and copper losses, i.e., a small transformer in order to obtain low iron losses and a large In order to choose a transformer based on economic crite- transformer in order to obtain low copper losses. The total ria, the cost price of transformers must be compared with transformer losses as well as the iron and copper losses for the cost of the losses. different transformer sizes are plotted in Fig. 3.

20000 3.1 Transformer Cost — Pilot. (kWWyear) •»•• Pen (kWh/year) The cost price of transformers is fairly linear. Fig. 2 shows •- Pfc (kWh/ycar) the cost in ECU for different rated apparent power, Sn, for the same series of standard transformers manufactured by S3 12000 •• ABB. %, 8000 • •

14000 •• 4000 - • 12000-•

10000 200 400 600 800 1000 8000-• 3 Sn CkVA) 6000-• Fig. 3: Total, iron and copper losses, in the trans- 40O0 -• former. 2000 •• 0 The calculated losses in the figure are based on the power 200 300 400 500 600 700 900 1000 production of a 600 kW Vestas wind turbine placed on a Sn OcVA) site with a wind characteristic according to Fig. 1. The Fig. 2: Costs of different transformer sizes. power production from the 600 kW wind turbine includes the active as well as the reactive power at different wind speeds. As can be seen in Fig. 3, the iron losses, Pfe, increase al- As can be seen in the figure, although the 630 kVA trans- most linearly with the transformer size. The copper losses, former has the lowest losses the 500 kVA transformer is the Pcu, are decrease with the transformer size. The total losses, most cost-efficient alternative. At least as long the electric- Pltot' are kept at a minimum at a transformer size equal to ity price is between 0.02 and 0.1 ECU/kWh. In the case of the rated power of the wind turbine. an electricity price below 0.02 ECU/kWh the 315 kVA transformer will be the most cost-efficient alternative. Worth mentioning is that the price for wind power gener- 3.3 Cost Comparison of Price and Losses ated electricity is approximately 0.046 ECU/kWh in Swe- As has been shown, a minimum of transformer losses is den today. found at a transformer with a rated power equal to the rated power of the wind turbine. Following the Swedish recom- mendation, in the example with a 600 kW wind turbine, a 4 TRANSFORMER OVERLOAD AND LOSS-OF- transformer with a rated power of at least 800 kVA should LIFE CALCULATION be used. The increased cost price for an 800 kVA transformer compared with a 630 kVA transformer is 511 ECU. The remaining questions are whether or not it is possible to The 800 kVA transformer will also have increased losses of overload the transformer and if it possible to overload the 638 kWh/year compared with the 630 kVA transformer. transformer without jeopardising the expected lifetime. Consequently, the 800 kVA transformer will not be the most cost-efficient alternative. 4.1 Transformer Overload In the case of a transformer with a rated power less than the At rated load and at an ambient temperature of 20°C the rated power of the wind turbine, the losses will increase. lifetime of a transformer is normally 30 years. Under these However, the cost price of a smaller transformer is lower. A operating conditions the relative ageing is 1. According to 500 kVA transformer costs 2 189 ECU less than a 630 kVA the International Standard IEC 354 "Loading guide for oil- transformer. The 500 kVA transformer has 1 560 kWh/year immersed power transformers" it is possible to overload a higher losses than a 630 kVA transformer. In other words, a transformer. The IEC 354 stipulates that the normal cyclic smaller transformer is less expensive but has higher losses loading not should exceed 1.5 times the nameplate rating compared with a transformer with a power equal to the on distribution transformers. The corresponding value for rated power of the wind turbine. short-time emergency loading is 2.0 times.

In order to select the most cost-effective transformer an Depending on the ambient temperature, transformers can economic optimum must be found. The question is whether withstand different degrees of overloading without high oil or not decreased losses will pay for the increased cost-price. and hot-spot temperatures. Looking at the ambient tem- One way is simply to calculate the annual cost of the peratures present in Sweden and the rest of the northern transformer. The annual cost A of the transformer consists part of Europe, a transformer will be able to withstand a of the capital cost and the cost of the losses and can be loading of 1.2 times the rated power. expressed by: A = CR + Plt0,E (2) Tab. 1 shows hot-spot temperature and the relative ageing where C is the cost price of the transformer, R is the fixed according to IEC 354 of a transformer at different ambient annual instalment, Pjtot are the tots' transformer losses and temperatures. The operating load is 1.2 times the rated E is the electricity price. The fixed annual instalment is power and the duration of the load peak is 24 hours. As based on a 25 years of repayment at an interest rate of 5%. pertains to high oil or hot-spot temperature the relative As can be seen in Eq. 2, the annual cost depend on both the ageing will exceed I. Note that the hot-spot temperature rate and the electricity price. should not exceed 140°C and the oil temperature should

The annual cost for different transformer sizes at different not exceed 105°C [5]. In the IEC, only the hot-spot tem- electricity prices are shown in Fig. 4. perature is calculated. The hot-spot temperature has a quicker response due to load peaks compared with the oil temperature. 2200 — Sn=31SkVA 2000 Sn=500kVA Tab. 1: Hot-spot temperature and relative ageing at -•-Sn=630kVA ' 1800 - - Sn=800kVA a load of 1.2 times the rated power 1600- Ambient temp. (°C) 20 0 -20 Hot-spot temp. (°C) 121 101 81 Rel. ageing (times) 15 1.5 0.15

As can bee seen, a transformer can withstand a load of 1.2

0.02 times the rated power, even at an ambient temperature of 20°C but with a high relative ageing as a result. At lower temperatures the relative ageing declines quickly. At higher Fig. 4: The annual transformer cost in ECU at different electricity prices and at a rate of 5%. loads and an ambient temperature of 20°C the hot-spot temperature will increase quickly with loss-of-life as a result. The relative ageing will be lower than what is shown • it is not reasonable to design the transformer for more in Tab. 1 if the transformer is placed outside and if the wind than 25-30 years length of life. is blowing. As we all know very well, high power from wind turbines only occur during high wind speed conditions. 5 CONCLUSIONS

The choise of transformer rating should be made based on 4.2 Loss-of-life calculation cost price of transformer, cost of losses and length of life If a transformer is overloaded or if the ambient temperature calculation. is too high, the oil temperature will also be high. For transformers designed according to the IEC 76, the relative The transformer should clearly not be larger than the rated rate of thermal ageing is taken to be equal to unity for a hot- power of the wind turbine. spot temperature of 98°C. The relative ageing is defined as: A 20% smaller transformer will be more economic and it is (4) not likely that it will have a length of life shorter than 30 years. where 0h is the ageing rate at the actual hot-spot temperature and ©h98°c is the ageing rate at 98°C. The loss of life 6 REFERENCES caused by month, days or hours of operation at a hot-spot temperature of 98°C is expressed in normal months, days [1] Dimensioneringsrekommendationer för anslutning av or hours. If the load and the ambient temperature are con- mindre produktionskällor till distributionsnätet stant during the period, the relative loss of life is equal (DAMP), Svenska Elverksföreningen, 1994, (in to V X t, t being the period under consideration. The same Swedish). applies if a constant operating condition and a variable ambient temperature are used. Generally, when operating [2] Newton, S.C., Clark, N.R., "Power Quality Measure- conditions and ambient temperature change, the relative ments on a 500 kW Variable Speed VAWT", Wind ageing rate varies with time. The relative ageing or loss-of- Energy, SED-Vol. 14, 1993, p. 149-154. life over a certain period is equal to: [3] Looijesteijn, C.J., "Disturbances in Electrical Supply l'f 1 N Networks Caused by a Wind Energy System Equipped with a Static Converter", Proceeding of the L = -|VdtorL = —Y V (5) BWEA 5, Reading, Berkshire, U.K., 23-25 March where n is the number of each interval and N is the total 1983, p. 303-317. number of equal time intervals. For example, if a transformer has a relative ageing of 1.5 times during 1 hour and [4] McCrea A., Jenkins N., "Experience of a 300 kW 0.9 times for 5 hours the relative ageing will be: Wind Turbine on an Extended 11 kV Network", Pro- 1x1.5 + 5x0.9 ceeding of a BWEA/RAL Workshop, Oxfordshire, = 1 times. U.K., June 1993, p. 1-12. 6 In other words, in this example the transformer will not [5] International Electrotechnical Commission, IEC 354, have any forced ageing. "Loading guide for oil-immersed power transformers", 1991. As concerns transformers for wind turbine operation there are some factors which would make it possible to overload the transformer. • during the time wind turbines work at part load (which is most of the time) the relative ageing will be much lower than 1. • during the hours the wind turbines are at a standstill the relative ageing will be 0. Since the hours per year when wind turbines are at a stand still or produce below rated power are many times the hours with rated power, the total relative ageing will not exceed 1. • during the time wind turbines produce rated power, the wind speed is at least 13-14 m/s. At these wind speeds, the transformer will be better cooled due to increased ventilation. • during the cold winter season in Sweden 70% of the yearly amount of wind blows. At that time of the year, the ambient temperature is very often below 0°C. FFATN 1997-48

Appendix 10

Continous Yaw-Control for Load Reduction

Tommy Ekelund CONTINUOUS YAW-CONTROL FOR LOAD REDUCTION

Thommy Ekelund Address: Control Engineering Laboratory, Chalmers University of Technology, S-412 96 Göteborg, Sweden

ABSTRACT This work's objective is to investigate the potential of continuous yaw-control for active attenuation of structural dynamic load-oscillations. In two design examples various structural modes are studied. The tower lateral bending mode shows the best potential for active load reduction. The results also indicate the importance of considering the system's dependence on the angular speed and position of the rotor in the controller design.

1 INTRODUCTION yaw control. For this purpose, we study a specific design case. As opposed to an earlier study, [2], the parameter For maximum efficiency, the rotor should be directed values are based on a real plant, namely a Swedish 400 kW towards the air flow. In medium to large sized wind- prototype machine [4]. Furthermore, the importance of the turbines alignment with the wind is predominantly achieved aerodynamic damping is also investigated. The work and actively, with an electric or hydraulic yaw servo. However, the models presented here are reported in more detail in [1]. the nacelle is mechanically parked most of the time. The servo is activated only when the mean relative wind- direction exceeds some predefined limits. 2 DYNAMIC MODELS If the yaw parking mechanism is stiff, then large dynamic loads appear in its components and the tower. Therefore, it The analysis is based on a set of mathematical models of is often better to make it flexible, for instance, with the plant. The equations of motion are derived with mechanical suspension devices. An alternative is to use the Lagrangian mechanics applied on two different models yaw motor continuously, instead of parking the nacelle. The consisting of flexibly interconnected rigid bodies. The same dynamic behavior as with a spring and damper nonlinear equations are linearized, for analysis and design. suspension can be obtained with feedback of the yaw angle. If the rotor speed is assumed constant, the resulting system The disadvantage of this concept is the increased demands becomes periodically time-varying. In a dense matrix form on the yaw servo. Continuous operation leads to increased the equations of motion can be stated as wear, and the ratings of the motor, for example, the maximum torque and speed, may have to be improved. M(r)q(0 + C(/)q(/) + K(?)q(0 = G(/)u(O + n(/) (1) Furthermore, additional or improved measurements may be where q(/) is a vector containing the degrees of freedom required. and u(0 is the control input. The mass M, damping C, stiffness K and input G matrices all have a periodic However, continuous yaw control has more potential than dependence on time. The last term n(/), representing the merely substituting a spring and/or damper. It may also be excitation from the wind speed and the gravity forces, has possible to actively attenuate other structural dynamic stochastic as well as deterministic components. A natural oscillations, since the yaw motion is dynamically coupled state realization of the linearized system is with the tower and the blades. The excitation of the structural dynamic modes comes from the wind turbulence and the periodic edgewise loads on the *(') = blades following from the revolution in a gravity field. The (2) resultant periodic loads on the nacelle and tower are significantly smaller in case of three blades, due to the symmetry of the rotor. Therefore, the incentive for continuous yaw control is larger for turbines with two (or one) blades. where x(t)=[4(r)T_/ATTq(0 I T Very little seems to be done in this direction. There are some studies on yaw control for power regulation. However, here it is assumed that the rotor should be 3 CONTROLLER DESIGN directed towards the wind. In [3] measurements are carried out on a 0 5.35 m yaw controlled turbine in a 12x16 m In the following, the torque applied by the yaw motor is wind tunnel. It is shown that feedback of the motor torque regarded as the control signal. This simplification is (hydraulic pressure) can improve the damping of the beneficial in this basic study, which merely aims at tower's lateral movements. examining the potential of the concept. The intention of this study is to investigate the potential for The primary objective of the controller is to minimize active damping of structural dynamic loads by continuous structural dynamic loads, given a certain restriction on the control torque. This constraint is considered since the rotation, so is the feedback. Likewise, the teeter dynamics achievable torque is limited. Furthermore, the control and the associated feedback gain are periodic with a full torque directly results in a torsion of the tower. The turn, due to the anti-symmetry of the rotor orientation after secondary objective is to keep the nacelle aligned with the 180°. Therefore these gains change sign with a period of wind direction. This is only important in a longer time 180°, whereas the yaw feedback is strictly positive. This scale, and has insignificant influence on the control signal indicates that time-invariant feedback of the teeter angle is variation during one single revolution. ineffective for teeter damping. The first and simplest method considered here is PD control of the yaw angle. The main reason for our interest in this '"••-. .••"* configuration is that the dynamics are equivalent to a spring **"••-. .-••'"* (P) and damper (D) in parallel, as in a mechanical suspension system. It also corresponds to the torsion in a flexible tower. Since both analogies are passive systems, it

is referred to as passive control or passive system. \

Yaw velocity "*-— •••• —-'' However, a controller based on a periodically time varying — Teeter velocity model ought to consider this dependence on time. Yaw angle ••'• \ ,...--' Therefore, it is natural to use the LQ design method, which — Teeter angle applies for such systems. As in time-invariant systems, the 0 BO 180 S70 360 control law is obtained from a Riccati equation. The Degrees difference is the absence of a stationary solution in the Figure 4.1 Periodic control law in case of no aerodynamic same sense. Here we seek the stationary periodic solution, damping. which is not derived as straightforwardly as its time- The improvement with the periodic feedback gain shown in invariant correspondent. Figure 4.1 is demonstrated in Figure 4.2. Compared with The two main differences between the passive scheme and passive feedback, the improvement is striking. The passive the LQ are that the latter is time varying and feeds back all concept only marginally changes the damping compared variables in x (2). The PD controller, which is stationary, with a stiff yaw system, even though the parameters are uses only information of the yaw motion. In practice it is chosen to increase the teeter damping. The improvement not necessary to measure all state variables. One may use a with LQ is achieved by means of larger yaw motion, which state estimator. However, most likely, it is motivated to is altogether acceptable since this motion is not directly measure at least the motion that should be attenuated. associated with any loads. The control torque is, in fact, significantly smaller with the periodic control law.

4 TEETER ANGLE

The rationale for making a teetered hub is to reduce hub loads originating from the wind gradient; external loads result in teeter motion instead of internal loads. Nevertheless, there may be an interest in restraining the teeter motion. For example, there have been failures as a Passive result of too excessive teeter motion [4]. — LQ Fixed yaw Two main physical circumstances makes it possible to Aerodyn.damping affect the teeter angle by the yaw motion. The first is the 5 inertia of the rotor, with respect to the teeter axis. This Revolution coupling is strongest when the rotor is horizontal, that is, Figure 4.2 Impulse response of the teeter angle. Only the when the yaw- and teeter axes are parallel, and vanishes in last curve, where the yaw is fixed has aerodynamic vertical position. The second phenomenon is the gyroscopic damping. torque, following from the rotation of the turbine being orthogonal to the applied yaw torque. The resultant Although the control torque is important for the loads in the gyroscopic torque is horizontal and perpendicular to the yaw mechanism and the tower torque, it is a poor measure drive shaft. of the actuator demands. A large torque can be achieved with a small motor, simply by using a large gear ratio. Most of the LQ weight functions studied here are diagonal Therefore, it is more adequate to study the control power and time-invariant. The objective is chosen to keep the Here, the power is limited to 1.5 kW. teeter and yaw motions as small as possible, and that the turbine should move in a reference plane orthogonal to the Since there is a desire to minimize the control power, it shaft. ought to be included in the LQ cost function. However, this involves an essential difficulty. The power is the product of The model we use here are of fourth order. The state the yaw angular velocity and yaw torque. It has to be variables are indicated in Figure 4.1, which shows an linearized, since the LQ methodology requires a linear example of how the four elements of a control law vary expression. The problem is that the power has a saddle during one turbine revolution. Note the difference between point in the operating point (the origin). Therefore, the the periods of the yaw and teeter feedback. Since the yaw partial derivatives with respect to speed as well as torque, dynamics, due to symmetry, are periodic with 180° turbine are zero. Hence, the linearized equation is identically zero, direction, we ought to consider an alternative reference and consequently of no use. trajectory. One suggestion is to determine this trajectory from the mean periodic variation that follows from the The aerodynamic damping, which is a very important external loads. In other words, we let the rotor self-adjust to phenomenon, is not considered in the simulations discussed some stable mean-path and then try to minimize the so far. Figure 4.2 shows that this damping is very deviations from this path. Thus, one need to decide what significant for the teeter motion. Our results clearly should be regarded as natural steady-state motion versus demonstrate that the improvement achievable by yaw transients. The former is determined not only by the wind control is highly dependent on the amount of aerodynamic field, it is also dependent on the yaw angle. Normally, the damping. Therefore, the potential for active attenuation nacelle is directed straight towards the wind. However, it depends on the blade characteristics as well as the mode of may be better to have a small misalignment so that, for operation (for instance if the blades are stalled or instance, the plane of rotation is sideways perpendicular to feathered). the wind direction. Next, we look at the response to a wind gradient. The The conclusion from these results is that the potential for applied wind speed corresponds to a constant wind speed controlling the teeter angle is highly dependent on the that increases linearly with height. Here the LQ control law aerodynamic damping. It is also evident that effective is modified, since the aerodynamic damping is included in control for reduction of transient teeter-motion should the model. The the control law in Figure 4.1 result in a consider the speed and angle of the rotor. large yaw misalignment. This follows from the wind gradient creating a steady-state force, which strives to turn the nacelle aside. This error can be reduced. For instance, the weight on low control torque can be decreased a factor 5 LATERAL BENDING 100. This significantly increases the feedback gain of the yaw angle. Thus, the nacelle is kept in position by a large The interaction between the lateral bending of the tower "spring constant". This is an inappropriate manner to and the yaw motion of the nacelle originates from the center compensate for a static torque-disturbance. The spring of gravity, of the nacelle together with the turbine, not constant should be used for the dynamic response. What we being placed on the yaw axis. As before, the LQ control law need is merely a constant torque, balancing the above is derived with constant costs. It still seems pointless to use mentioned static side-turning aerodynamic force. Integral time varying cost functions. Furthermore, the integral of the feedback of the yaw angle is the most obvious way to yaw angle, rather than the angle and its velocity, is achieve this compensation. penalized. Figure 5.1, where state variables are listed, shows an example of the control law. The period of the gains being one half rotation follows from the symmetry of the two-bladed rotor. From the tower point of view, after a 180° turn the rotor is in an equivalent position. This symmetry also leads to gains that do not change sign, although there is a small exception in the feedback gain of the bending angle. Therefore, it is reasonable to assume that the damping can be improved by a constant control law.

300

200

100

1-100 Figure 4.3 Teeter angle, impulse response with linear wind t \ / y gradient. The passive controller has the same parameters as t -200 Yawveloc. Tower veloc. before. > -300 Yaw angle -400 Tower angle Yaw integral Figure 4.3 shows that it is very difficult to reduce the teeter -500 motion. Only the LQ controller including integral feedback -600 "'-•- y manages to accomplish this to any noteworthy extent. -700

However, the required effort, is very large, with exceptional Roto angle (degiees) demands on the controller bandwidth. Figure 5.1 Control law with integral feedback of the yaw The above results show the problem in opposing the natural angle. teeter motion originating from the wind gradient. It requires the gyroscopic torque to balance the flatwise force The impulse response in Figure 5.2 shows improved produced by the wind gradient. Furthermore, the damping, both with a passive controller and the time gyroscopic torque works in the desired direction (parallel to varying control law in Figure 5.1. In all three simulations the teeter axis) only during a small part of the revolution. the aerodynamic damping is included in the model. On the other hand, we want the rotor to be self adjusting However, the aerodynamic forces on the blades are much and level out load gradients. This is the reason for using a too small to have any significance on the heavy bodies in teetered hub in the first place. Therefore, the objective in motion. the above example is slightly improper. Instead of forcing the rotor back to the plane perpendicular to the wind time-varying feedback and measurement of the bending motion. The teeter angle is significantly more difficult to control. The reason is the aerodynamic damping, which makes the effort required to change the natural teeter motion very large. i. None of the above results are conclusive, but it is reasonable to assume that the plant parameters have to change considerably to affect these basic results.

10 12 14 16 18 20 Considering the present results, the method of reducing Revolution structural dynamic loads by yaw control cannot be Figure 5.2 impulse response with moderate control action. recommended as a standard solution. In some cases it may be advantageous, but it is not altogether clear which these Although the tower oscillation declines at equal rate with are. Therefore, it is desirable to investigate how the system the two controllers, there is a distinct difference in phase. parameters affect the performance and the cost efficiency. The passive system has approximately the same phase as One question arising from the results concerning the teeter the uncontrolled fixed-yaw system. This important motion is if there are any cases where the aerodynamic difference represent two separate modes of oscillation, see damping is sufficiently low to consider continuous yaw Figure 5.3. The LQ controller uses the inertia of the nacelle control. The aerodynamics are less important for the lateral and rotor in a dissimilar, more efficient, manner compared tower-bending. with the passive PD controller. The controller effort is less than 10 percent of the passive controller. The magnitude of It is also of interest to explore the dependence on the the actuator power is, with the LQ feedback, comparable mechanical system parameters. It is advisable in such with the rated value of the present servo. studies to use the basic models applied here. The model order can presumably not be further reduced. However, the '(DGQQCX) QOQCD analysis can be clarified by normalization, which reduces the number of parameters. The results are based on the assumption that the yaw torque is controlled. There is another interesting possibility for hydraulic yaw systems, namely to use a valve to regulate Figure 5.3 Illustration of the motion of tower and nacelle: the hydraulic oil flow. This would be like controlling a leftLQ, right PD. viscous damping coefficient, or using time-varying derivative feedback. What makes this idea interesting is that A controller with less weight on the control effort no effort is put into the system. Thus, the ratings of the yaw attenuates the tower resonance faster. In the design of the motor can be lower. LQ-controller above, the penalty on control torque was chosen by comparing the resulting control power with the rated value of the existing servo. The PD parameters are selected in order to accomplish the same tower damping as 7 REFERENCES the LQ controller. We can conclude that there is a potential for reducing the [1] Ekelund T. (1997), Modeling and Linear lateral tower-bending with continuous yaw control. Quadratic Optimal Control of Wind Turbines. Compared with the teeter motion, the aerodynamic damping Control Eng. Lab., Chalmers Univ. of Techn., S- is less important for the performance. It is also possible to 412 96 Göteborg. Technical Report No. 306, improve the damping with a conventional PD controller. ISBN 91-7197-458-x. However, for effective attenuation, it is still preferable to feed back the rotational angle when deriving the control [2] Ekelund T. (1996), Yaw Control for Active signal. Damping of Structural Dynamics. Proc. Europ. Union Wind Energy Conference and Exhibition, Göteborg. 6 CONCLUSIONS [3] Ulén E. (1993), Wind Tunnel Tests ofae 5.35 m Yaw Controlled Turbine. The main result of this study is, in general terms, improved understanding of the limitations and requirements The Aeronautical Research Institute of Sweden, associated with active damping of some structural Report no: FFA TN 1993-20. resonances by continuous yaw control. [4] Vattenfall (1995), Final report Evaluation ofLyse The lateral bending mode of the tower shows the best wind power station 1992-1995. potential. This motion may even be reduced with a passive Vattenfall AB, 162 87 Vällingby, SWEDEN. yaw system. However, the associated yaw torque is considerably smaller when the damping is achieved with FFATN 1997-48

Appendix 11

A New IEA Document for the Measurement of Noise Immission from Wind Turbines at Receptor Locations

Sten Ljungren A NEW IEA DOCUMENT FOR THE MEASUREMENT OF NOISE IMMISSION FROM WIND TURBINES AT RECEPTOR LOCATIONS

Sten Ljunggren Address: Dep. of Building Sciences, Kungl. Tekniska Högskolan, S-100 44 Stockholm, Sweden

ABSTRACT A new IEA guide on acoustic noise was recently completed by an international expert group. In this guide [1], several practical and reliable methods for determining wind turbine noise immission at receptor locations are presented: three methods for equivalent continuous A-weighted sound pressure levels and one method for A- weighted percentiles. In the most ambitious method for equivalent sound levels, the noise is measured together with the wind speed at two locations: one at the microphone and the other at the turbine site. With this approach, the turbine levels can be corrected for background sound and the immission level can be determined at a certain target speed. Special importance is attached to the problem of correcting for background noise and to techniques for improving the signal-to-noise ratio. Thus, six methods are described which can be used in difficult situations.

1. INTRODUCTION occur during a measurement. For this reason, an interpolation scheme must be used. A starting point for the work of the IEA group has been the practice developed by local and national authorities In addition to the requirements on broadband sound, for specifying limits for the noise from wind turbines. usually expressed as an equivalent sound level or an A- Such limits are usually expressed as an immission level, weighted percentile, the requirements often contain a that is a leve] to be measured in a relevant noise sensitive penalty for audible tones. Such tones are defined in area. Immission limits can, in principle, be verified by different ways in different countries. The presence of measurements in the same way as noise immission limits different methods is a severe drawback for an for other outdoor sources such as road traffic, aircraft, international industry, which prefers a single method. industries etc. However, a major problem when measuring noise immission from wind turbines is the Thus, a major part of the work of the IEA group has been presence of the wind. Another important factor is the fact devoted to these three issues. that the immission levels specified by the authorities tend to be low. Thus, the background noise is often of the 2. MEASUREMENT OF EQUIVALENT SOUND same order of magnitude as the sound level from the LEVELS turbine, which makes reliable measurement results difficult to achieve. According to the main method, the noise level is measured with a microphone at a height of 1.5 m above An immission limit is usually specifed for a certain wind the ground together with the wind speed at the turbine speed at the turbine. However, that wind speed will never site and also close to the microphone. The instrument configuration is illustrated in Figure 1.

rr ? T Anemometer ! Microphone Anemometer 2

Figure 1. Illustration of instrument configuration for measurement of equivalent sound levels

The sound leve! is measured simultaneously with the The level of the sound from the turbine(s) is finally wind speed at the two anemometer positions and plotted as a function of the wind speed at the turbine site, averaged over the same time intervals (each interval 1 - and the level at a target wind speed, often 8 m/s at 10 m 10 min, number of intervals > 10). These measurements height, is determined, see Figure 3. are carried with the turbine or turbines in operation. dB(A) Before and/or after these measurements the background sound (with the turbine(s) parked) is measured and averaged over similar time intervals together with the 44 a wind speed at the microphone. It is now possible to plot the total sound level from the first measurement and the ** background sound alone as a function of the wind speed at the microphone, see Figure 2, and to determine the 36 sound level from the turbine(s) alone.

32 10 m/s dBIA) US Figure 3. Determination of the turbine sound level at the target wind speed (here 8 m/s). a a a n a Combined noiss • a a a a The measurement of the wind speed at the turbine site x / Background noise must be undertaken with caution. In the case of a single X X 36 turbine, the speed should in the first place be obtained from an anemometer at hub height or from the electric 32 output together with the power curve. If an anemometer 5 6 7 8 9 10 m/s is used, it should be placed according to Figure 4.

An anemometer at a height of 10 m and placed in front of Figure 2. Plotting of combined sound level (sound the turbine tower can also be used. from turbine(s) and background) and background

sound level as a function of the wind speed at the In the case of a wind farm, the anemometer should be microphone. placed in a position which is relevant for the sound generation, see the two examples of Figure 5. Wind direction Two methods, which are simplified with respect to the wind speed measurement, are also described in the document. The very limited use of these methods in practice is pointed out in the document.

3. MEASUREMENT OF A-WEIGHTED PERCENTILES

As an alternative to a measurement of an equivalent sound level, one or several A-weighted percentiles can be determined. Figure 4. Recommended position of the turbine site anemometer for the case that the anemometer is In this case, the sound from one or several wind positioned at hub height. D is diameter of wind turbine(s) is measured at receptor locations together with the wind speed, obtained from an anemometer positioned turbine rotor. on a meteorological mast at the wind turbine or wind Y farm site. The background sound is also measured as a function of the wind speed obtained from the same Wind direction Y anemometer. A procedure for obtaining a conservative estimate of the levels of the turbine(s) alone is described. x o , Microphone Anemometer This method is used if the limits are expressed in Y percentiles and in particular where the noise immission limits are related to ambient sound measured previously Y at the receptor location. The technique is applicable for measurement of any A-weighted percentile. However, it should be noted that the percentiles L^lO» LA90 and 5 are the ones most often used in practice.

4. MEASUREMENT TECHNIQUES FOR CASES Wind OF LOW SIGNAL-TO-NOISE RATIOS direction

O Noise immission measurements around a wind turbine Anemometer Y Y Microphone Y will often be influenced by background sound. If the equivalent sound level of the turbine together with the Figure 5. The upper part of the figure illustrates a background is more than 3 dB over that of background case where the wind speed should preferably be alone, the level of the turbine can be determined directly. measured at the closest turbine, while the lower However, it is a common experience that the signal-to- part illustrates the case where the wind speed noise ration is lower. Then one or several of the should be measured at an undisturbed position in following methods can be used: the vicinity of the group of turbines. • change of time of day for measurements from, for short term spectra are obtained with time weighting 'F' instance, day to night, and an averaging time of 0.2 - 0.5 sec. • repositioning of microphone to a position at the same distance and sound propagation conditions but with a For boths methods, the analysis of the tonality comprises lower background sound level, four steps: • use of a secondary (and larger) wind screen, • identification of the lines in the spectra as either • use of a large vertical measurement board, see Figure tones, masking noise or neither, 6. • calculation of masking level, Two approximate methods are also described. One uses • calculation of tonal noise level, measurements at a lower wind speed together with a • evaluation of tonality. correction for the change in source strength. The other uses measurements at a reduced distance together with an 6. ACKNOWLEDGEMENTS appropriate correction. The IEA document [I] has been developed through a series of meetings with participants from different a 1.8 m countries participating in the IEA R&D agreement. The

Microphone members of the expert group have been: position S Bent Andersen, Denmark Microphone position A T. James Du Bois, USA Nico van der Borg, Holland Maurizio Fiorina, Italy Helmut Klug, Germany Mark Legerton, UK Sten Ljunggren, Sweden B. Maribo Pedersen, Denmark. In addition, valuable information has been received from Jergen Jakobsen, Denmark. Figure 6. Position of the microphone on the large measurement board. Front view. Microphone position A The work was partly founded by the EU commission is recommended when the board is used to reduce the under contract No JOR3-CT95-0065. influence from sources or reflecting surfaces shielded by the board. Microphone position B is recommended when 7. REFERENCES the board is used to reduce the wind induced microphone noise. [1] S. Ljunggren (ed), Recommended Practices for Wind Turbine Testing. 4. Acoustics. Measurement of Noise 5. EVALUATION OF TONES Immission from Wind Turbines at Noise Receptor Locations (in print - the document will be available Two methods are presented for the analysis of the tones. through the members of the IEA standing Committee). According to method I, at least five RMS spectra are [2] O. Fégeant, Measurement of Noise Immission from obtained with a bandwidth of typically 2 - 5 Hz and an Wind Turbines at Receptor Locations: Use of a Vertical averaging time of 1 - 2 min. For method II at least 50 Microphone Board to Improve the Signal-to-Noise Ratio. EWEC 97. FFATN 1997-48

Appendix 12

Measurements of Noise Immission from Wind Turbines at Receptor Locations: Use of a Vertical Microphone Board to Improve the Signal-to-Noise Ratio

Olivier Fégeant MEASUREMENTS OF NOISE IMMISSION FROM WIND TURBINES AT RECEPTOR LOCATIONS: USE OF A VERTICAL MICROPHONE BOARD TO IMPROVE THE SIGNAL-TO-NOISE RATIO

0. Fégeant Address: Dep. of Building Sciences, Kungl. Tekniska Högskolan, S-100 44 Stockholm, Sweden

ABSTRACT The growing interest in wind energy has increased the need of accuracy in wind turbine noise immission measurements and thus, the need of new measurement techniques. This paper shows that mounting the microphone on a vertical board improves the signal-to-noise ratio over the whole frequency range compared to the free microphone technique. Indeed, the wind turbine is perceived two times noisier by the microphone due to the signal reflection by the board while, in addition, the wind noise is reduced. Furthermore, the board shielding effect allows the measurements to be carried in the presence of reflecting surfaces such as building facades.

1 INTRODUCTION 2 DIFFRACTION EFFECTS

Acoustical outdoor measurements are often made The board is oriented so that the microphone is mounted difficult by the action of the wind on the microphone on the side facing the turbine, considered here as a point system, the existence of ambient noise and the presence radiator. For any source located in the semi-infinite of scattering obstacles (building facades, etc.) around the space Vi (Figure 1), the corresponding pressure at the measurement position as they contribute to lower the microphone is the sum of the incident, the reflected and signal-to-noise ratio. Mounting the microphone on the tiie diffracted fields. surface of a rectangular vertical board is expected to be an efficient solution to improve it as the reflection by the rigid board adds 6 dB to the signal-to-noise ratio incident sound wave whereas the microphone is sheltered from the background sources located behind the plate. Furthermore, the wind velocity, viz. the pseudo-noise generation, is reduced at the microphone due to the wind profile blocking effect of the board on the flow.

microphone Nevertheless, diffraction occurs at the plate boundaries, affecting not only the reflection of the incident noise but ground also the sheltering effect. This study was designed to evaluate the scattering effect of a board placed vertically Fig 1. Description of the problem above a ground as well as the wind noise generation. As the microphone is mounted on the rigid board, the of interest. For the sake of convenience, this point will reflected and incident pressure are equal, and this leads be will be denoted MoPt in the rest of the article. to a pressure doubling effect (+ 6 dB) while the diffraction effects are responsible for the deviation from pressure doubling.

2.1 Deviation from pressure doubling The expression of the pressure at the microphone has been derived by writing the problem in its integral formulation and using the Kirchhoff s assumptions. The diffraction field appears then under the form of a sum of •0.8 -0.6 -0.4 0 2 D 0.2 0.4 0.S 0.3 several integrals which might be evaluated asymptotically by the method of stationary phase as described by [1]. Thus, in the simplest case, i.e. at the Fig 2. Standard deviation of departure from centre of a square plate of half edge length a, the pressure doubling plotted over the board surface expression of the total field under normal incidence is given by: 2.2 Ground reflection The ground induces reflection for both the acoustical .K zi eika Jkajl waves launched by the turbine and by the diffracted (1) Pi •Jit -Jkä n-Jl ka field. Nevertheless, this latter is neglected due to its poor contribution and the scattering effects of the direct and reflected fields from the turbine may be assessed The first term at the right shows the pressure doubling separately by considering two plane waves impinging on due to the reflection, the second the diffraction by the the board with different angles of incidence. edges and the third the one produced by the comers. It appears that diffraction depends only on the Helmholtz's 2.3 Sheltering effect number (kd) and the bigger it is, the less the diffraction The microphone perceives only the diffracted fields of effects. Eqn.(l) also shows that the edges and the the sources located in the volume V2 and, thus, the corners generate respectively cylindrical and spherical shielding effect is also maximum at Mopt as the waves. Due to the simplicity of the solution obtained by diffracted fields are equal on the two board sides. This the method of stationary phase, it is quite easy to obtain effect is especially important when the measurement a presentation of the diffraction field over the board location allows reflections to occur from building surface. Indeed, by computing eqn.(l) over the frequency facades situated behind the microphone. By considering range (100, 5000 Hz) sampled with a frequency a plane incident field impinging on one side of the board resolution of 10 Hz, the standard deviation of the and its image by the board plane impinging on the other departure from pressure doubling may be calculated at side, the shielding effect is defined as the difference each frequency and summed over all the frequencies. between the sound pressure levels induced by both fields Figure 2 shows the result of such a simulation repeated at the microphone. Figure 3 shows the result of a for numerous board points. It appears that some calculation made with an incident field composed of two positions minimise the diffraction effect and that, under non-correlated waves of incidences 10° and -10° normal incidence, the centre and the board axis of respectively impinging on a board of dimensions symmetry are maxima of diffraction. The position of the (1.8mxl.5m). It is observed that the sheltering is already best minimum depends of course on the angle of 10 dB at about 150 Hz when the microphone is incidence, the size of the board and the frequency range positioned at MoPt(0.45, 0) while this effect, assessed at two board sizes, (1.8m x 1.5m) and (0.9m x 0.75m) the centre is lower and more irregular. during the day and for wind velocities contained in the range (4m/s, 7m/s). The velocity is given at 1.5 m above the ground which is the board centre height. The 25

0pu t 5 pressure perceived by a microphone mounted on the 20 board and sheltered by a 9.5 cm hemispherical 15 JM windscreen is compared with the 'free microphone' 1u SO 1 > 10 value given by a microphone embedded in a 9.5 cm 5 - 5 spherical windscreen. On the board, the microphone was v' 0 t located at the centre and at the point MoPt (0.45, 0). 10 100 1000 Frequency, Hz 3.4 Measurement results F/g 3. Shielding effect assessed atM (—) and at opt It has been observed that the wind noise generation is the board centre (—) strongly dependent on the board size, on the flow velocity and on the frequency of interest. Thus, it was straightforward to express it under the dimensionless 3 WIND NOISE GENERATION form adopted by Strasberg [3]. And it has thus appeared that the measurements satisfy eqn.(2) with a standard The wind noise arises from the action of the wind on the deviation of 3 dB. measurement system itself and is composed of a non- radiated aerodynamic noise due to the internal flow 201g(- (2) velocity fluctuations and of the near field aeroacoustical where p^ is the pressure in the third octave band f and p noise of the measurement system. According to previous is the flow density. St, the Strouhal number, is defined studies [2] [3] [4], the first component is dominant in the by St = £D/V where D is a typical dimension of the wind and the second in a low turbulence rate oncoming board or of the windscreen and V is the flow mean flow. velocity. A and B are two constants which vary with the measurement point. Finally, as expected, it has been 3.1 Flow description observed -that the gain is slightly better when the As the flow passes the board, bands of vorticity are microphone is placed at the stagnation point. generated at the edges of the obstacle and, at some distance behind, they roll up and form the vortex street -40 also called the wake [5]. In the windward, the flow i velocity decreases as it approaches the board surface and i Ll even vanishes at the stagnation point which characterises -60 w a division of the stream. The pseudo-noise is then expected to be lower at this point compared to any other oa -80 PHi! point of the board. J P -100 11m 3.2 Measurement description 0.1 1 10 100 Outdoor measurements have been carried out to provide Strouhal number, £D/V an estimation of the attenuation given by the use of a Fig 5. Outdoor dimensionless wind noise board measurement system instead of a free microphone. ( o ) Measurements with the free microphone They were performed in the neighbourhood of ( O) Measurements with the boards Stockholm in July 1996. The records were made with Figure 6 shows the gain of signal-to-noise ratio obtained Concerning the diffraction effect, it has been seen that it by using the board instead of the free microphone. It is might be reduced at suitable positions on the board seen that this gain is greater than 5 dB over the surface. For those points, the sheltering for a board of frequency range [20,1000 Hz]. (1.8mxl.5m) is greater than 10 dB above 150 Hz. Thus, this study shows that the use of a board is advantageous

15 for outdoor measurements when one is dealing with low signal-to-noise ratios or with reflecting surfaces around the measurement location. / CQ 10 \ -a j \ _c ~J s .g / \ O 5 ACKNOWLEDGEMENTS

This project was partly funded by Contract N° JOR3- 10 100 1000 CT95-0065 from the EU-commission and partly by Frequency, Hz Contract N° PI 887-2 from the Swedish National Board for Industrial and Technical Development (NUTEK) Fig 6. Gain of signal-to-noise ratio

6 REFERENCES 4 DISCUSSION AND CONCLUSIONS

[1] J.Stamnes, Waves in Focal Regions: Propagation, Dimensionless laws giving the wind noise generation for Diffraction and Focusing of Light, Sound and Water both the board and the free microphone have been Waves. Adam Hilger, Bristol and Boston, 1986. derived, allowing the results to be extrapolated to [2] M.Strasberg, Dimensional analysis of windscreen different board sizes or wind velocities. Nevertheless, noise. Journal of the Acoustical Society of America, these laws could have suffered from the influence of the 83(2)(1988)544-48. short velocity range [4-7m/s]. It has been shown that the [3] M.Strasberg, Nonacoustic noise interference in board should be placed normal to the wind direction in measurements of infrasonic ambient noise. Journal of the order to maximise its blocking effect. Then, a suitable Acoustical Society of America, 66(5) (1979) 1487-93. microphone position on the board surface is the [4] S. Morgan, R. Raspet, Investigation of the stagnation point, which may be located a bit over the mechanisms of low-frequency wind noise generation board centre (about 10 cm) due to the logarithmic wind outdoors. Journal of the Acoustical Society of America, velocity profile. Both indoors and outdoors, it has been observed that the board measurement system might lead 92(2) (1992) 1180-83. to a higher background level compared to the free [5] A.Fage, FJohansen, On the flow of air behind an microphone at middle and high frequency. It is difficult inclined flat plate of infinite span. Proceeding of the to conclude if it arises from the generation of pseudo- Royal Society of London, Al 16 (1927) L70-97. noise by the board, i.e. a high sensitivity to the flow turbulence, or if it is the consequence of the reflection of the background noise by the board. Anyway, the addition of 6 dB to the incident signal counteracts largely this negative effect and makes the board measurement system still interesting for improving the signal-to-noise ratio. FFATN 1997-48

Appendix 13

The Social Impacts of Wind Power

Karin Hammarlund THE SOCIAL IMPACTS OF WIND POWER

Karin Hammarlund Department of Human and Economic Geography School of Economics and Commercial Law, Gothenburg University S-411 80 Göteborg, Sweden

ABSTRACT This paper attempts to compress the multiplicity of perspectives often found under the denomination 'social aspects of wind power'. The main part of the research so far focus on opinion surveys. This paper outlines the main acceptance problems and sum up the acceptance situation in Sweden in comparison to Holland, Denmark, Germany and United Kingdom. Public acceptance is connected to public participation in the planning procedures as legally prescribed or in different ways enhanced. The study gives a general picture of how differences in planning practice and legal systems effects the acceptance of windpower. When the authorities give priority to other land use interests and treat wind power as a threat to recreational and environmental interests, it is obvious that policy concerning the use of a landscape contains alot of values and contradictions. I will try to provide you with some examples of this. The pros and cons of renewables will appear at different geographical and administrative levels. This result in a situation where immediate visual disadvantages will shade long term benefits of sustainable energy supply. We must begin to understand the social impacts of not accepting windpower. The benefits of renewables often require a global perspective in order to stress the fact that every energy system based on non renewable fules must come to an end. We do not need to argue about if it takes thirty, sixty or ninety years to complete these resources. Instead we now need a supreme effort to develop the best alternative energy system. 1 INTRODUCTION 2. IDEOLOGICAL CONTRADICTIONS

Public relations are an important part of national 2.1 The defenition programs for wind energy development. One aspect of Social stands for human society, it's organization or public relations is to find out what people think about quality of life [1]. Hence, the social aspects of windpower wind power relative to fossil fuels. Another aspect is to involve not only permit processing but also how it find out the public attitude towards a specific wind farm in effects the quality of life of the people concerned. the nearby environment. Public surveys concerning Competing perspectives on land use stem from the nature attitudes towards wind power have already been carried of land development plans and policies for conservation. out, many in Britain, but also in the Netherlands, It is therefor essential to clarify what values in a Denmark, Germany, Sweden and other countries. landscape we are trying to conserve and for whom.

Public acceptance of wind power can be looked on as a 2.2 The organization of society superfluous concern since the pros and cons are obvious. The organization of society is held in place by different But are they obvious? Advocates of wind power do not social institutions with a task in many cases to resist understand why the effects of wind power on the landscape rapid change. This can be seen as a predicament when are more problematic than the effects of other working with wind power development. Wind turbines constructions and development So it seems that wind result in a rapid visual change of the landscape in power can be harmless to some and a presumed disaster to comparison to most constructions. It takes less than a day others. to errect a turbine extending 30, 60 or more meters up in the air. The effects are immediate. Opinion surveys i Europe show that acceptance of wind power development is more connected to the use of wind In contrast, modern farming methods, and the large scale turbines than the aesthetic evaluation of them. Hence, one production systems they require, gradually alters the of the most important prerequisites for a positive cultural landscape. Planners and the general public do not experience of wind power is for the turbines to be in normally react to this type of gradual but extensive operation as much as possible. The visual impact of change. There is an explicit need to cultivate the land spinning turbines seem to be a central problem for which prevents us from automatically questioning or planners but the practical solution for the public after discuss the aesthetic impacts of fanning. There is however siting. no explicit need for renewable energy yet.

I will discuss some of the social aspects of wind energy 2.3 The effects development and draw some general conclusions from In Sweden we have the unique "Allemansrätten" which different opinion surveys. Futhermore, I will discuss the gives the public recreational access to the countryside. In use of public surveys. I will also discuss different ways of southern Sweden large grain fields prevent this access due experiencing and making use of the landscape. This to the fact that people can not just walk over the fields. involves ideological contradictions manifested through This prevented access is not something that the public, legislation and planning procedures. Further, I will argue living in this part of the country, thinks about. They that the categorization of attitudes will help us to usually do not stop to argue about long term visual, understand the makings of attitudes. Finally, I suggest biological or other effects of modern farming methods. that we look at the benefits of wind power, and understand the consequences of not accepting it as a renewable source There is no acute acceptance problem with the long term of energy. irreversible change of cultural landscapes caused by modern farming methods, pesticides or air pollution. Why then is there an acceptance problem with wind turbines. even if the effects of wind power are quite reversible? The extent on what we recognize as effects since nature can reason for this might be that it is easier to discuss not speak for itself. something that is as immediatly visual as one or many wind turbines. Also, the benefits of wind power to a It is possible to diminish environmental effects through public accustomed to a conventional energy system, short term economic attractiveness. This is evident in based on fossil fuels (at what seems to be a rather low todays energy system based on fossil fuels. In this cost), are not yet apparent. Furthermore, nuclear power context it may seem remarkable that renewables are not plants, strip-mining and waste-disposal often do not yet seen as economically attractive and advocates of wind directly affect coastal areas of scenic beauty. power feel almost provoked when planning authorities and the general public speak about the environmental To protest against land use which has irreversible and effects of wind power. Researches are trying to make the wide spread long term effects requires an insight into environmental costs (external costs) of fossil fuels these effects. Even specialists can fail to acquire such explicit in order to promote the attractiveness of knowledge. To concentrate on the store of fossil fuels and renewable alternatives. how long they will last us might seem more beneficial than it is to recognize that every energy system based on It is easy to assume that wind turbines should stand a fair non-renewable energy must come to an end. chance of being looked upon as environmentally acceptable. However, the benefits of wind energy often These were a few examples of how we can look at or fail to require a global perspective. The disadvantages of wind recognize the environmental effects depending on: energy, on the other hand, are placed in a local setting • the visual evidence, where people experience visual and audible effects from • the pace of change, wind turbines. The general attitude toward wind energy is • the custom of use however often found to be positive, and many wind • the accessibility of knowledge, enthusiasts would like to leave it at that • and to a certain extent logical error. I have choosen to study the makings of local opinion 2.4 The values concerning wind turbines, in order to illuminate what Environmental and cultural values are implicit in the other factors besides economic attractiveness makes it landscape, but they become explicit through people. possible to view a wind turbine as something acceptable. Consensus is difficult to achieve since we make use of the landscape in different ways. It is possible to group attitudes towards wind turbines according to what activity people are engaged in.

Farmers are dependent on rural survival and might look upon wind power as a contribution to their subsistence. Conservationists who wish to preserve existing rural landscapes and heritage might see wind turbines as a visual threat. Urban people who escape to their summer residence for recreation and recuperation might find the sound from wind turbines disturbing.

Local and regional planning authorities have a hard time stipulating the rules for what is to be seen as significant effects on the environment. The effects depend to a certain 3. OPINION OF WIND POWER

3.1 The general attitude and experience When reviewing the results from different surveys of opinion concerning wind power in countries like the Netherlands, Denmark, Sweden and the United Kingdom it is evident that the experienced effect on the landscape is closely related to the personal attitude toward wind energy in general. A positive attitude in general toward wind energy promotes a positive experience of the effects of wind turbines after siting. It is, however, essential to actively promote and enforce a positive attitude among the public when planning for wind turbines in their near by environment, because a wide support for wind energy does not guarantee local support for wind turbines. [2]

Fig 1. A sense of co-operation 3.2 Information and co-operation The results from opinion surveys show that good planning including public co-operation can make the 3.3 The function difference between a positive or a negative experience of There are hesitations among the concerned public during wind turbines in the close-by environment. Unknown the introduction phase of each wind power project. actors who enter the local 'arena' must do so carefully and However, the experience of an operating wind turbine this applies to most development plans. Throughout the turns most fears and hesitations into fascination and planning process of a wind power development it is delight. This fascination and delight is connected to the essential to inform the public. Preferably through a novelty of wind turbines and might disappear as the continuous dialogue which makes it possible to treat number of turbines increase. It is evident that an questions as they arise. insensitive handling of the planning process for wind power location can also diminish positive reactions. To involve people in a wind energy project means involving them in the process of change of their own The experience of spinning turbines enforce the function neighbourhood. It is of greater importance to grasp the and use of wind energy in the landscape. Wind turbines way people perceive their landscape than it is to, for earn their rightful place in the landscape when people example, adjust the design of a wind turbine to anticipated perceive them as useful. It is obvious that the public is aesthetic preferences. The public can provide information more concerned about production statistics than the visual on matters of importance to the development which are impact after siting of wind turbines. . not always apparent in land use plans. If a wind power developer engages people and there is a sense of co- 3.4 The time aspect operation, this might prove essential to further All discussions about the public attitude toward wind development of a site. turbines must be clarified by the time aspect, that is pre or post siting. Attitudes are connected to direct experience or lack of experience of wind turbines in operation. Figure 2 shows approximately the number of turbines now located in Germany(D), Denmark(DK), the Netherlands(NL), the United Kingdom(UK) and Sweden(S). Fig 3. The experience of sound and noise

Fig 2. Registered turbines in 1996 In conclusion, important factors deciding the public opinion of wind turbines in the landscape are: 3.5 The sound and personal relation • pre- or post siting - the time aspect It is obvious that people can detect the sound from a wind • the general attitude turbine at a long distance but the experience of this sound « a sense of co-operation varies greatly. Some people argue that it is most of the • the personal experience time impossible to distinguish the turbine sound from the • the function of the turbines sound of the wind. If a person is somehow connected to a • personal relation to the turbines (share holder, owner, wind energy project (shareholder, owner etc.) he/she is summer guest, farmer etc.) likely to experience the sound from that/those wind • the information provided turbine(s) producing as something reassuring. Summer • co-operation residents on the other hand are sometimes more likely to experience the sound as something disturbing the peace 4. THE USE OF PUBLIC SURVEYS they seek in the countryside. It is clear that the irritation threshold is set at very different levels. Hence, set noise 4.1 On the national and regional level standards in dBA can be insensitive of the individual Some of the major formal ways by which the state can experience. express it's will in order to directly influence the behaviour of people are: • legislation • taxation • subsidies • economic policy • information campaigns However, the ability of the state to influence development is always uncertain because there is always a world of quite unforeseen reactions. [3] These unforeseen reactions might become target for 5. ATTITUDES national surveys launched in order to understand what really happend. An improvement would be if peoples 5.1 Defenition of attitudes reactions were integrated in to the planning process from the very beginning. " Attitude is a psychological tendency that is expressed by evaluating a particular entity with some degree of favor There is always a scope of various interpretations when it or disfavor". [5] comes to legal rules, in particular when certain reservations are included from the beginning. In Sweden We are predisposed to evaluate things as positive or this becomes evident since the majority of environmental negative. Some argue that we do not have an attitude until regulations state restrictions based on the fact that on we respond evaluatively to something. It need not be an going activities should not be seriously disturbed. Hence, object, e.g. a turbine, that we respond to it could be in a system of functional sectorization there is a high risk things related to it, e.g. sound, nuclear power, etc. In this that different land use is allocated under competition and context it is important not only to critically examine the this results in a power relation rather then a balanced type of questions used in an attitude or opinion survey, solution.[4] but also how we analyse the results.

Direct experience is multi-dimensional in ways that 5.2 Cognition, affect and behavior numbers, graphs and pictures never can be. Planners in Attitudes are not directly observable but can be inferred countries with some wind power experience must abstract from observable responses to a stimuli. As a stimuli it is from peoples experience of this wind power. It might possible to use questions about ones experience and views become evident that wind energy does not stand in in relation to wind turbines, it is possible to show oppostion to other land use. pictures of wind turbines or one could visit a wind power location. Whatever we choose it is important to 4.2 On the local level understand that the answers we get are dependent on the As mentioned before it is necessary to distinguish stimuli we use. One could say, "as you ask you will be between those who have a direct experience of wind answered". energy and those who are confronted with a development plan. The only aspect that seems to be of equal The experience of a spinning turbine is multi-dimensional importance to the time perspective is how people make which means that visualizations of planned projects use of the landscape. might cause more damage than do good because a picture can not enforce the functional aspects of turbines. As we It is possible to categorize the different actors according have seen before, the functional aspect is crucial if people to the type of activity they perform in the landscape. are to experience wind turbines as useful. Furthermore it is illuminating to add the amount of time each category devotes to their activity for example the People's attitudes can be divided in to three clases: farmer, the summer resident and the turist. This • cognition categorization may act as a guiding principle and an • affect objective in the otherwise quite subjective priciples of • behavior. landscape conservation. It is important to clarify what we The cognitive category contains thoughts that people are protecting and for whom we are protecting it. have, for example about wind turbines. The affective category consists of the feelings and emotions that people have and the behavioral category contains people's actions.[5] This division clarifies that attitude surveys need not be 6.2 Conclusion decisive of behavior. However, cognitive and affective responses are connected to behavioral response. Hence, if Research on the social impacts of a new technology like we do not know what thoughts and ideas that people have wind turbines require an interdisciplinary approach. You about wind turbines we will surely not understand their need to understand the requirements of the technology in actions. order to mediate between the views of different concerned actors. The solution seems to lay in good mediators between actors with a too focused view. observable inferred observable cognitive Most landscapes are products of aesthetic or economic stimuli • attitude affective decisions made centuries ago. We as individuals best enjoy the landscapes that we have grown up with and behavioral inevitably our views of a landscape is tainted by our subjective opinion. There are significant differences between the landscape perceptions of professional Fig 4 Attitude as an inferred state [fromEagly&Chaiken, 1993] planners and the general public. Hence, it might be better to ask the actual users of a landscape to evaluate its The distinction between cognitive, affective and quality. [7] behavioral responses provides an important conceptual framework which helps us to understand different In whatever way we see and experience wind turbines there components of attitudes.[5] is a certain underlying truth. If we refuse them and their effects on the landscape we must be aware of the fact that 6. THE BENEFITS non-renewable energy (fossil fuels, nuclear power) may not seem to have geographically extensive visual effects, 6.1 Are there any visual benefits of wind but instead they have irreversible environmental effects turbines? extending far beyond our imagination. Landscape architects talk about 'mystery' as a landscape factor which involves a promise to leam more. 7. REFERENCES

"Something in the setting draws one in, encourages one [1] Longman Dictionary of Contemporary English, to enter and to venture forth, thus providing an Longman, 1978. opportunity to learn something that is not immediatly [2] Opinion Surveys 1977-1995 apparent from the original vantage point" [6] M. O'Hare, "Not on My block you don't": facility siting and the strategic importance of compensation. Public Wind turbines can add to the potential of new information Policy 25,1977. in the landscape and to the human inclination to explore. G. Marks, D. von Winterfeldt, "Not in My Back Yard": Mystery and exploration is important for extending one's Influences of motivational concerns on judgments about cognitive map. Also, wind turbines can help restore a risky technology. J. of Appl. Psych. 69,1984. ruined industrial landscape because they can symbolize J. N. Pinder, M. A. Price, Measurement of noise and the hope for a better environment. Wind turbines can help assessment of likely disturbance due to noise from a 20 people to orient themselves in the landscape, and there kW windturbine generator at Milton Keynes. University are more examples. Southampton,1988. K.Hammarlund, 'Vindkraft i Vallby', Inst. för Kultur- geografi och ekonomiskgeografi, Lunds Universitet, 1988. M. Wolsink, The social impact of a large wind turbine, [5] A.H. Eagly, S. Chaiken, The Psychology of Attitudes, Environmental Impact Assessment Review, 32, 8 (4), pp. Harcourt Brace&Co, 1993. 301-312. 1988. [6] S. Kaplan, R. Kaplan, Cognition and Environment: M. Wolsink, Attitudes and expectancies about wind Functioning in an Uncertain World, Praeger N.Y., 1982. turbines and windfarms. Wind Engineering, 13 (4), pp. [7] J. Douglas Porteous, Environmental Aesthetics: ideas, 196-206. 1989. politics and planning, Routledge, 1996. K. Hammarlund, 'Havsbaserad Vindkraft i Nogersund', Karlshamnsverkets Kraftgrupp/Sydkraft.1990. K. Hammarlund , "Varför satsar man på mindre och medelstor vindkraft i Sverige?', Intervjuundersökning kring privata satsningar på mindre och medelstor vindkraft KV AB, 1992. AIM, Föreningen af Danske Vindm0llefabrikanter. Holdningsundersögelse. Job nr. 6254, 1993. K. Hammarlund, 'Havsbaserad Vindkraft i Nogersund', Karlshamnsverkets Kraftgrupp/Sydkraft, 1993. B. Young, Attitudes towards wind power A survey of opinion in Cornwall and Devon, ETSU/W/13/ 00354/ 038/REP, 1993. Wolsink, M., Sprengers.M, Keuper, a., Pendersen, T.H., Westra, C.A., Annoyance from wind turbine noise on sixteen sites in three countries. EWEC'93, Proceedings of an international conference held at TravemUnde, Germany. pp. 273-276, Stephens& Associates, 1993. E. Esslemont, Cemmaes Windfarm, Sociological impact study: Final Report ETSU W/13/00300/REP.1994. K. Bishop, A. Proctor, Love them or loathe them? Public attitudes towards wind farms in Wales. Report of research commisioned by BBC Wales, 1994. J. Jordal, J0rgensen, Samfundsmasssig vasrdi af vindkraft. Delrapport: Visuelle effekter og st0j fra vindm0ller - kvantificering og vaerdisaetning. AKF, 1995. J. Munksgaard, A. Larsen, J. Jordal-J0rgensen, J. Rahlbask Pedersen, Samfundsmaessig vaerdi af vindkraft. Delrapport 2: Miljömässig vurdering av vindkraft. AKF, 1995. K.Hammarlund, Public Acceptance, Final Report: Evaluation of Lyse Wind Power Station 1992-1995, Vattenfall, 1995. [3] T. Hägerstrand, What about People in Regional Science, Regional Science Association Papers, Vol. XXIV, pp 7-21, 1970. [4] T. Hägerstrand, Landet som trädgård, in Naturresurser och landskapsomvandling, Rap. Bostadsdep. och FRN, pp 7-20, 1988. FFATN 1997-48

Appendix 14

Evaluation of the Nordic 1000 Prototype

Staffan Engström, Göran Dalen, Jan Norling EVALUATION OF THE NORDIC 1000 PROTOTYPE

Staffan Engström n, Göran Dalén2), Jan Norling3>, Anders Andersson4*, Göran Ronsten5), Ulf Johansson69, Natan Gothel/K Hans Ganander8>, Erik Rudolpfu9)

1) Nordic WindpowerAB. Box 968, S-191 29 Sollentuna. Sweden, Fax +46-8-754 99 -17 2) Vattenfall AB. S-162 87 Stockholm, Sweden. Fax +46-8-739 55 62 3) Vattenfall Utveckling AB, S-810 70 Älvkarleby. Sweden, Fax +46-26-83670 4) Vattenfall AB. Box 88, S-620 20 Klintehamn. Sweden, Fax +46-498-489 164 5) FFA. Box 11 021, S-161 11 Bromma, Sweden, Fax+46-8-25 34 81 6) Vattenfall Utveckling AB, S-162 87 Stockholm. Sweden, Fax +46-8-739 60 76 7) Harmonizer Power Quality Consulting AB. Dalv. 57. S-141 71 Huddinge. Sweden. Fax +46-8-662 90 92 8) Teknikgruppen AB. Box 21. S-191 21 Sollentuna, Sweden, Fax +46-8-444 51 29 9) Ingemansson Technology AB, Box 47321, S-100 74 Stockholm, Sweden, Fax +46-18 26 78

ABSTRACT

The Nordic 1000 is a 1000 kW two-biaded, horisontal axis, variable RPM stall controlled wind turbine. The project started in January 1993. The prototype was erected at Vattenfall^ Wind Power Station at Mäsudden on Gotland in April 1995. The commissioning has proceeded without any major problems. As of August 1997 Nordic 1000 has been operated grid-connected during 11800 hours and produced 2.8 GWh. The paper presents results from the evaluation, made to the same standard as the other CEC Joule 2 WEGA II projects. Keywords: Wind Turbines, HAWT, Stall Regulation

1. THE NORDIC 1000 PROJECT Table 1. Operational statistics from June 1995 to August 1997 The Nordic 1000 project is one of the JOULE II/WEGA II projects. The main design work started in January 1993. Availability Operation Net prod. The design is described in ref. (1). The prototype turbine (%) (%) (MWh) was erected in April 1995 at Vattenfall's Wind Power Jun.-Dec. 95 - 58 462 Station at Näsudden on the island of Gotland in the Baltic. Jan. 96 81 67 91 It has been operated since June 1995. Experiences from the Feb. 59 46 82 commissioning process are described in ref. (2). Mar. 36 26 33 Apr. 32 26 44 The evaluation has been managed by Vattenfall according May |_ 83 66 118 to the same standard as the other WEGA II projects. For this purpose Vattenfall Utveckling AB has installed a data Jun. 83 50 63 acquisition system. It consists of four PC-based computers Jul. 83 66 120 that measure 50 channels at 32 Hz. All raw data are stored Aug. 80 61 71 on CD-ROM. Each disc also holds 1 and 10 minute statistic Sep. 65 63 94 files for all channels. Oct. 75 62 155 Nov. 94 84 215 This report presents the main results of the evaluation. Dec. 92 82 178 Jan. 97 76 70 141 Feb. 61 59 227 2. OPERATIONAL STATISTICS Mar. 79 67 217 Apr. 74 68 169 The operational statistics cover the whole period from the May 31 28 62 first grid connection in June 1995 to August 1997. This Jun. 96 76 96 means that problems encountered during the Jul. 90 75 100 commissioning have influenced the results presented in Aug. Table 1. The main cause for down-time and operational 79 58 51 restrictions has been the frequency converter and its Total 72" 61 2789 communication with the control system. Serial units will operate without frequency converters, see Table 2. 1) Based on the period January 1996 to August 1997

DUBLPAPUXK 4. POWER CONTROL QUALITY

In order to evaluate the power control quality of the Nordic 1000 an analysis has been carried out, including time histories, amplitude spectra and histograms covering the power control behaviour.

In general the analysis reveals a behaviour which fulfils expectations. Above rated wind speed the power is effectively limited lo rated power, with a m.iximum for instantaneous power excursions of around 25% of rated.

The frequency analysis reveals a spectrum with expected although low peaks due to the wind shear at 2.' (twice per rotor revolution) and, to some extent at 4P, 6P and 8P. In Fig. 3 an example of an amplitude spectrum is shown.

Wlap 20 2P 1

\ 1S 10 j Figure 4. The Nordic 1000 prototype at Näsudden. 6P x /v A 3. POWER PERFORMANCE

The power performance evaluation of the Nordic 1000 prototype has been based on wind data from an existing Figure. 3. Example of an amplitude specimmfor 145 m meteorological mast and a purpose built 58 m mast. The latter was erected when it became obvious that the active power at 15.8 -19.1 m/s wind speed. wind speed measurements of the old mast did not fulfil today's standards of accuracy. 5. YAW QUALITY The wind turbine configuration evaluated has a pitch angle The yaw error has been evaluated during roughly one of 2.2°, two stall strips per blade and 9.5 m of vortex month of operation. In general the analysis reveals a yawing generators on each blade. The objective of the stall strips is behaviour that fulfils expectations. Below rated wind speed to improve the stall behaviour. The vortex generators serve there is a maximum of the 10 minute mean yaw error of 2 to increase the performance at wind speeds below rated. degrees. Above rated wind speed the error increases to around 4 degrees. See Fig. 4. The power performance was evaluated according to the recommendations of the IEC and is reported in ref. (3). The measured power versus wind speed curve appears in Fig. 2.

* . ..

;• i * • *. • a soo ••• I N*l pow«r (KW1. 080 V HI - ltd - Contracted POw«r curv* A/0'- to is ao 25 JZL • >. • •

Wind speed [m/s] Figure. 4 Yaw error as a function of wind speed Figure 2. Power versus wind speed for Nordic 1000 measurements on top of the meteorological mast. during 970402-970615. 600 s mean values, 970402-970501. Flapping moment at rotor centre {kNm) It can be noted that the mean values of the yaw error as 1200 calculated from the meteorological mast measurements and from the observations on top of the nacelle differ only 4 degrees at all wind speeds. However, the standard 1000 deviation of the yaw error based on mast observations is considerably higher, which is a consequence of the distance between the wind turbine and the mast. Thus, one may draw the conclusion that the standard deviation observed on the nacelle is more representative than that based on the mast measurements. They are 3 and 5 degrees respectively at low wind speeds and 3 and 4 degrees at high wind speeds.

6. RESONANCES

In the Campbell diagram, see Fig. 5, a comparison between S !0 15 multiples of rotor speed excitations and some of the main component frequencies is shown. Note that the first and the Edge moment at rotor centre (kNm) second tower eigenfrequencies (Tower 1 and Tower 2) are well below 2p and above 6p, as intended.

200

•200

•400 20

Yaw moment on top of tower (kNm)

Figure 5. Comparison of multiples of rotor speed excitations and component frequencies. •200

•300 7. MAIN LOADS

Loads during normal operation are illustrated in Fig. 6 which depict the average values of mean, max. and min.for some important parameters during consecutive 10 rotor revolutions on April 2-4 1997. Note that the data points form quite distinctive patterns. Figure 6. Mean, max. and min loads as a function of In general the analysis reveals loads that are tow and well wind speed. Values calculated as averages during within foreseen values. The dynamic behaviour of the consecutive 10 revolutions of the turbine. Date turbine is benign. No unforeseen amplifications of motions 970402 11:00 - 970404 01:45. or loads have been found. Table 2. Main data Nordic 1000 8. GRID INTERFERENCE

Prototype Serial version During the initial measurements of grid interference Type of design soft unacceptable high values of the 13th and I lth harmonics Control principle stall were noliced, probably due to a resonance. During these Hub teeter measurements four out of the six filters of the frequency Orientation upwind conversion system were connected. The last two filters had Turbine been installed for fine-tuning of the system. After Turbine diam. (m) 53.0 connection of the last two filters and minor adjustments, the resonance was reduced resulting in levels of harmonics Rot. speed (r/tnin) 12-25 17/25 below 3.5% in total harmonic distortion at the 690 V Tip speed (m/s) 33-69 46/69 busbar. Airfoil LM2. NACA 63.4xx , FFA-W3 Machinery 9. NOISE Gear-box 2-stage planetary with integral turbine shaft bearings The noise level of the turbine has been measured according to the IEA recommendations No 4. Acoustics. The apparent Ratio 1:51.7 1:60.6 A-weighted sound power level was determined to 104 Electrical system Induction with Induction 2-speed dB(A) re I pW. Prominent tones probably originate from frequency the second stage of the planetary gear-box. With these converter totally eliminated, the total noise level of the turbine would Power (kW) 1000 1000/250 be reduced by 4 dB(A). The manufacturer is working on a Tower solution to reduce the level of the tones. Type steel-shell

Diam. top/base 1.9/2.6 10. CONCLUSIONS (m) Weights • The design, construction and commissioning of the Machinery and 42 Nordic 1000 prototype has proceeded without any turbine (ton) major problems. Total excl. 93 foundation (ton) • The evaluation has been completed with a general Electrical production conclusion that the technology works as intended. (Mean wind speed 2100 2400 7 m/s, MWh/yr.jl • In general loads are low and well within foreseen values. The dynamic behaviour of the turbine is benign. No unforeseen amplifications of motions or loads have been found.

• Major modifications foreseen on the serial version are reduced noise emission and the use of a two-speed generator directly connected to the grid, resulting in lower cost and higher energy production.

11. REFERENCES

(1) B. Bergkvist, S. Engström and G. Dalen: Commission- ing/Design Experience with the Nordic 400 and 1000 kW Prototypes. 5th European Wind Energy Association Conference and Exhibition. 10-14 October 1994, Thessaloniki, Greece. (2) B. Bergkvist, S. Engström and G. Dalen: Evaluation and Commissioning of Nordic 400 and 1000 Prototypes. 1996 European Union Wind Energy Fig. 7. Nacelle and machinery of Nordic 1000.1. Conference. 20-24 May 1996, Göteborg, Sweden. Blade. 2. Hub. 3. Platform. 4. Teeter bearing. 5. (3) G. Ronsten: Power Performance of the 1 MW Wind Turbine Nordic 1000. FFATN 1997-31. Teeter stop. 6. Hydraulic cylinder for blade-tip. 7. (4) E. Rudolphi: NWP 1000 Prototype. Measurements of Planetary gear-box. 8. Brake disc. 9. Secondary Noise Emission from a Wind Turbine according to IEA shaft. 10. Safe-set coupling. II. Generator. 12. Hoist. recommendations 4. Acoustics. Ingemansson 13. Rescue equipment. 14. Oil cooler. 15. Nacelle. Technology 1997-05-16. 16. Hatch. 17. Yaw bearing. 18. Yaw drive. 19. Hydraulic unit. FFATN 1997-48

Utgivare Beteckning

Flygtekniska Försöksanstalten FFA 1997-48 Box 11021 Datum Sekretess Ex nr 161 11 BROMMA November 1997 Öppen 7- Dnr Antal sidor 342/97-23 28 + App 1 t.o.m. App. 14 + docsida Uppdragsgivare Projektnr Beställning

NUTEK VE-0180 P2939-2 Vindkonsortiet 117 86 STOCKHOLM Dnr 4F6-97-04489

Rapportens titel V indkraftskonsortiet MIUU CTH Sammanställning av föredrag vid EWEC'97 i Dublin Författare

Anders Östman Granskad av Godkänd av

Sven-Erik Thor Chef, Vindenergi Sammanfattning Den Europeiska vindenergiföreningen, EWEA, arrangerar vart tredje år en konferens och utställ- ning, European Wind Energy Conference EWEC. I år, 1997, arrangerades EWEC'97 i Dublin. Vart tredje år arrangerar dessutom Europeiska Unionen EUWEC, sin vindenergikonferens. Konfe- renserna är förskjutna ett och ett halvt år inbördes. Det innebär att det i Europa arrangeras vindkonferenser var 18:e månad. I föreliggande rapport redovisas erfarenheter från EWEC'97.

Nyckelord Vindkraftskonsortiet, EWEC'97, Vindkraft

Distribution NUTEK VKK Styrelse FFA

Antal ex/ex nr 1-7 8-18 19-70 Printed at FFA Printing Office. Bromma